It sounds like they are referencing a bit of science mixed with humor. As is typical on Reddit. My interpretation is that they’re implying that extreme weight loss like the one shown by the 4chan screenshot, is probably harmful to one’s body at even a molecular level, by mentioning telomeres. Telomeres are protective caps at the end of chromosomes that shorten as we age or undergo stress, including from poor diet and extreme weight changes. The phrase "trans fats all over his telomeres" isn't a scientifically accurate statement. While poor diet can contribute to inflammation, oxidative stress, plaque buildup in the cardiovascular system, and general cellular damage, which in turn can accelerate telomere shortening, it's not a direct action of trans fats themselves. Instead, the relationship between diet and telomere length is more indirect, through overall cellular health and damage over time.

The phrase is being used humorously to suggest that a bad diet (with trans fats) could potentially worsen this damage, even down to a deep, molecular level. In other words, they’re using biochemistry terms loosely to make a point that such a drastic transformation can’t be healthy.

As for the Christian Bale reference, Bale famously lost a dramatic amount of weight to play an emaciated insomniac in The Machinist (2004), where he dropped to around 55 kg (121 lbs). Shortly after that, he gained a significant amount of muscle mass for Batman Begins (2005). If an actor like Christian Bale can drastically lose and gain weight, theoretically, anyone could too. But there could be negative health effects, especially if done in an unhealthy way.

Hope that clears it up!