I have a Masters in History and this is a fun thought exercise on historical memory so bear with me:

Lots of random literary or cultural references were preserved in early cartoons like Looney Tunes (1930-1969) which was in rerun syndication at least until the early 2000s and would have had viewers as old as grandparent age watching with their grandkids (so if it’s a grandparent taking their grandkid to the movies to see the first cartoons, they could have reasonably been born during the Civil War). Cartoons always have references in them to appeal to the adults that inevitably will also be watching and many would have been reading books, known oral history or known about general world news headlines so there would have been common tropes in literature that folks were familiar with.

One reference I can think of in particular that I knew about as a child from watching Looney Tunes that I would never have really known about otherwise was a scene where one of the characters reenacts a hypothetical journey by Dr. Livingstone, a Scottish explorer who went missing in 1866 while exploring central Africa. Apparently the words “Dr. Livingstone, I presume” were supposedly uttered in 1871 when journalist Henry Morten Stanley went on an expedition to track him down (although there are no physical records to this, it was omitted from Livingstone’s journal and Stanley’s were destroyed). Nonetheless, the public was captivated by the story, so much so that it was preserved in a children’s cartoon, made many decades later that children and their parent or grandparent would understand that reference.

My assumption is that as an American classic novel with a reputation as having been an amazing book its own right would have been familiar enough for a general American viewing audience that it made its way into multiple forms of media that was later consumed by wider audiences, hence familiarity with the general plot and themes without ever reading the book. It would have also been a common story to rehash/recycle in a new light over time like Josef Conrad’s Heart of Darkness, published 1899 and the movie Apocalypse Now, released in 1979. We all know the line “the horror, the horror!” but may not have known that it was uttered by fictional Kurtz in a Victorian Era book. Books and stories about exploration and adventure have been part of the America zeitgeist since the country’s inception.