I feel like both of these things rely on the same coping mechanism.
In the dark ages, life was difficult, disease everywhere, high child mortality rate, and even being alive meant working very hard. It’s a nice comfort to tell yourself that it doesn’t matter what happens to your body because it’s not your REAL self, just a fragile vessel, and things will get better eventually because even if you’ve got the super triple-plague and a ton of infected feet blisters, you can distance yourself from it by mentally assigning your “true self” to a soul or other detached part of you, away from your body. That was the christian belief at the time: it doesn’t matter what kind of pain or trauma happens to your body because it’s only temporary, and your soul is what matters most.
FAs do a similar thing, othering their bodies as if it’s just an ancillary part of them and not as important..a way of compartmentalizing the daily pain and aches of obesity and ignoring it. Their “real selves” are defined, not in their souls, but on their social media, their internet footprint, their twitter expressions. The REAL them is this non-physical, digital version of themselves that is unfettered by an aching and heavy body. Having to get offline and suffer through their day as a physical being is just an unfortunate part of reality, one that can be ignored the moment they shove a phone or computer back in their faces to express their “true” selves.
In that respect, I can see how their body might feel like a house...a dirty, messy one you drag yourself home to, which you hate and wish you could move out of, but which you can drown out and ignore for the night by sitting in front of the telly or playing video games.
The interesting thing is they detach from their bodies, while at the same time they are worshipping and fetishizing their bodies.
It's one thing to be detached from your body if you're in a historical situation like you've described, or if you just don't care that much about it. There are lots of people who are "trapped" in difficult bodies, like paraplegics or people with rare degenerative diseases, that find fulfillment in life through cerebral pursuits and hobbies. For instance, I knew a kid who had one of those rare disorders where he was essentially paraplegic and needed special equipment to be able to communicate. Luckily, living in a wealthy country in modern times, he was able to study pretty much any subject he wanted online. He was a pro at MMORPG games and found a tight friend group there.
That kid didn't really have a choice, but I also think there's people who just aren't super into their bodies or into sensual experiences. Especially among "very online" people, they find their hobbies and their passions which are not physical, and maybe (hopefully) they just put enough energy into taking care of their bodies to stay healthy-ish. I also remember in one of Dan Savage's books, where he talks about the then-early FA movement, he describes a woman at an FA event who says she really only cares about reading. The prospect of becoming so fat that she lost some abilities wasn't that distressing to her, as long as she could still read. Being beautiful wasn't important to her. Being healthy, or perceived as healthy, wasn't important to her.
That woman was certainly showing some very unhealthy thinking, but there was still an internal logic-- my mind and body are separate, I don't get any particular joy from my body, so I don't really care what happens to it. It's a bit fucked up, but it makes a kind of sense.
FAs on the other hand-- they use this language to distance themselves from their bodies, and yet pour ALL their energy into trying to convince themselves and others that their bodies are healthy and beautiful. It's not "I'm not particularly invested in health because it just doesn't matter to me, so I don't really identify that much with my body." It's "My body is separate from me and it is not my identity, BUT it is also my entire identity and there is nothing else in my life that is important to me, BUT I'm actively antagonistic towards health, AND the idea of health being a virtue is oppressive, BUT being fat isn't unhealthy anyway, AND ALSO my body is beautiful and desirable and I will never shut up about it!"
I've said it before, it's like cult thinking. It's contradictory and illogical and keeps its practitioners in a constant state of self-policing. But my question is: to what end? Who's benefitting from this? Purveyors of fast food and fast fashion, I suppose-- but I honestly don't think those industries NEED this lobby to keep going. It's just unhealthy people encouraging others to ruin their health, for what? This whole thing is psychopathic.