The non sarcastic answer is that Popsy appeals to a certain demographic - and the clothes are easy to manage for someone with chronic ill health as they don't have to deal with tight fabrics, zips or buttons etc. There is a huge co-morbidity between certain chronic illnesses - as as Fibromyalgia, POTS, EDS and other auto-immune conditions - and neurodivergence which also explains why a lot in that demographic are also autistic or have ADHD etc.
Part of ASD and particularly ADHD is a tendency to overshare - this also comes from the fact a lot of people don't believe us when we tell them we're ill, or neurodivergent so we feel the need to go into excess detail. We also struggle to read social cues.
As for self diagnosis - if you existed within the realm of neurodivergence and these kind of chronic illnesses you'd learn that getting in front of the appropriate medical professionals is very difficult, with long waiting lists or barriers to getting referred in the same place. Not having an official diagnosis doesn't mean you don't have the condition though. Obviously. And as my own GP said - before I had to wait almost two years for an official Fibromyalgia diagnosis 'if it walks like a duck and quacks like a duck, chances are it's a duck and we'll treat it like that'.
Autistic/ chronically ill people often have to become experts in their condition. This is doubly true if we're also advocating for our children who are also likely to be unwell as it's all genetic. While there are no doubt some people who just love attention etc, an awful lot of us are coming from an informed and experienced place. And by informed I don't just mean TikTok videos.
Living with chronic illness takes over your life and affects almost every aspect of it. It's not surprising we talk about it - we don't have the wealth of other experiences and opportunities healthy people have.