Obesity
is an eating disorder. It's one that's killed orders of magnitude more people than anorexia or bulimia ever have.
> People don't know what a genuinely healthy body and diet/fitness regimen look like
That's in large part because even healthy people can't agree on what that looks like. Regardless, many people don't seriously care about eating healthy because otherwise they'd simply use the internet and quickly find some diet that actually works (pretty much any of the "fad" diets are better than the Standard American Diet).
> thinness (even extreme, unhealthy thinness) is popularized and praised as a sign of beauty and willpower because so many people are trying to lose weight and so few are trying to gain.
This effect can't be totally discounted, but is it really that or is it because being fat actually kinda sucks? The average person is not attracted to obesity, and obesity can lead to trauma that then leads to things like anorexia. Ironically, obesity is also caused indirectly by poor mental health.
Contrary to this idea that thinness is somehow this societal evil, obesity is more normalized than it's ever been. That's only going to do the opposite of getting people healthy. When was the last time an unhealthy image of thinness was promoted in the mainstream? Besides the fashion industry, which most people don't take seriously or even pay attention to, I can't think of even one.