Long-standing cultural traditions and being far more ecologically familiar with dolphins would explain why certain orcas hunt dolphins but none hunt humans. It is the possible theory of mind aspect that raises more questions than answers. But I'll expand a bit.

One of the most important things to know about orcas behaviour is that they are extremely cultural animals.

As is stated by biologists Luke Rendell and Hal Whitehead in their 2001 paper "Culture in whales and dolphins":

The complex and stable vocal and behavioural cultures of sympatric groups of killer whales (Orcinus orca) appear to have no parallel outside humans, and represent an independent evolution of cultural faculties.

Orcas are usually very selective predators. Though orcas as a whole prey on a large amount of different species, they belong to very different populations, and each of these populations has its own unique culture. Many of these populations/communities do not interact or interbreed with each other. Their diets are a major part of each of these cultures. So, due to these cultural reasons, orcas are often unwilling to change from their pod's/community's diet. They aren't really free to choose and switch to different prey species on a whim.

Thus, the most comprehensive theory on why orcas do not desire to eat humans supported by marine biology can be summed up as follows. Orcas learn what to eat from their mothers. These dietary preferences are passed down generations (culturally transmitted) within an orca population. Specific diets form a major part of the cultures of each unique orca community/population. Culture seems to be very important to orcas, and thus orcas will rarely stray outside of the diet they are taught to eat by their podmates. In addition, even to mammal-eating orcas, humans likely simply do bear any resemblance to the various mammals that these orcas are familiar with.

Culture also determines the different social structures, ceremonies/rituals/traditions, social norms, play activities, and more amongst different orca populations.

Now that this has been established, there is more to discuss about the phenomenon of wild orcas attempting to give their food to people.

Basically, the study, which was published in a comparative psychology journal, is yet another piece of evidence that orcas as well as various other cetaceans do seem to view humans as fellow social beings that they could attempt to communicate with. In these encounters involving them attempting to share food with humans, they may be trying to essentially experiment on us to see how we react to them, thus learning more about our behaviour.

Since food-sharing with other orcas often seems to be important to orcas from many populations, perhaps it is not all that surprising that they would try to do the same for us. They seem to be exhibiting a form of cross-species generalized altruism towards humans in these scenarios.

Jared Towers, the executive director of Bay Cetology and leading author of this paper, also mentions that orcas may have theory of mind. This means that they could have the ability to detect and recognize our mental states. This also means that they may recognize that us humans also have our own perspectives and that we also may also be another highly social and intelligent lifeform.

Orcas do not really interact with many other species in this way, usually either ignoring, harassing, or killing other sea creatures. So it seems that orcas see humans a bit differently, perhaps being more similar to themselves. Thus, at times, there are individual orcas that appear to be highly curious about us and what we are doing in their world.

There are a multitude of reasons for the orcas to engage in this behaviour, as stated by the authors of the paper, "Offering items to humans could simultaneously include opportunities for killer whales to practice learned cultural behavior, explore or play and in so doing learn about, manipulate or develop relationships with us. Given the advanced cognitive abilities and social, cooperative nature of this species, we assume that any or all these explanations for, and outcomes of such behavior are possible. These cases suggest that societies of generalized reciprocity are prevalent in some populations of this species and indicate that as in humans, sharing is a cultural by-product used by killer whales outside of their own species to explore relationships within their respective environments."

There are multiple other cases of orcas attempting to share food with humans/boats/cameras not included in the study, such at least one from Australia and this one from Antarctica.

Perhaps the most famous and extensive relationship between humans and orcas was Old Tom's pod forming a cooperative relationship with whalers in Eden, Australia. Perhaps this cooperative relationship was actually started by these orcas approaching and offering humans their food and the humans reciprocating many years ago, or vice versa.

Now that it has been established the orcas both are highly dependent on their respective cultures and also may have theory of mind in regards to how they relate to humans, we can start hypothesizing about the populations of orcas that hunt other dolphins, such as those in the Eastern Tropical Pacific (ETP).

The orcas in the ETP are particularly relevant to this question, because they may have a rather generalist diet that includes other dolphins, as well as rays, sharks, fin fishes, larger whales, and sea turtles (though there could still be multiple different orca "ecotypes" in the ETP).

Since 2019, there has been a sharp increase of tourists booking tours to have the opportunity to swim with the Eastern Tropical Pacific orcas off of Baja California (e.g. out of La Ventana), to the point where local biologists have proposed much stricter regulations. And yet, nobody has been harmed by these orcas that hunt other dolphins.

In fact, one of the accounts of an encounter in the study even mentions that an orca spat out the eyeball of a bottlenose dolphin right in front of the face of a swimmer, which is quite wild.

Other oceanic dolphins are also very intelligent and social creatures. Orcas can learn their calls and may be able to communicate effectively with them once they do. It would be rather surprising if orcas, with their apparent theory of mind, saw themselves as more similar to humans than to other dolphins. And yet, some populations of orcas, including those that have been documented expressing generalized altruism towards humans, ultimately still hunt other dolphins.

So I wonder if in certain orca cultures, other dolphins are seen as acceptable prey, while in other orca cultures, other dolphins are not seen as acceptable prey for reasons beyond diet preferences. Perhaps the orcas that do hunt other dolphins may partially recognize similarities between their species, but still see other dolphin species as being on a lower level than orcas in their culture, and thus acceptable prey. And perhaps for orcas that do not hunt dolphins, dolphins are perceived as being too similar to orcas in their culture to become acceptable prey, even if these orcas are still not interested in socializing with other dolphin species.

The fact that orcas in many populations can be quite xenophobic, both towards orcas from other populations as well towards as other dolphin species, adds another layer to the mystery of why they may still try to initiate contact with and exhibit generalized altruism towards humans.

Perhaps one of the reasons they are more curious about us and are more willing to form some kind of relationship with us is not only because they may perceive us as being on a cognitive level of similar to themselves, but also because we come from a world (dry land) totally alien to them, while they have frequently seen other aquatic species, including other dolphins, for millions of years, and are highly ecologically familiar with them. Some wild orcas, such as "Digit," have appeared to be interested in learning more about humans.

In an old documentary about the famous intentionally-stranding orcas at Punta Norte, Argentina, orcas appeared to befriend local park ranger Roberto Bubas. Dr. Jane Goodall states in the documentary that the orcas wanted Bubas in the water with them because they wanted to experience him and were curious about him as an individual. Bubas may also represent another world (dry land) that these orcas could not experience themselves.

Humans may represent a world that they cannot ever access. Thus, orcas could see forming a relationship with humans as being potentially beneficial to themselves.