I don't follow this particular community but I do feel conflicted about some parts.
How people choose to live and how they feel good about themselves is their business and I wouldn't go to someone and point out whether I think they are unhealthy because what do I know? I'm not their doctor. However, while aiming for accepting and loving your body, I feel like some people swung to the other extreme and some of them actually think wanting to lose weight/adopting exercise and healthy habits is like betrayal to body positivity? This is so strange to me. If someone is content living the way they do and they aren't harming anyone else (like encouraging the unhealthy habits for their children), go and prosper. I think clinging to coping mechanisms or cliquey expectations is damaging, and it undermines the actual attempts at trying to create a supportive, accepting environment for all.
Thin privilege does exists; in ways that being thin is encouraged regardless of methods and circumstances of achieving it (e.g. I was at my lowest weight when I couldn't eat because of anxiety; I was constantly sick and could barely stand to be in other people's company but I kept getting compliments despite being very much dead behind the eyes and it showed) and how majority of the consumer goods are catered to a fairly limited type of figure. I have met a bunch of overweight women and men who can easily run rings around me when it comes to endurance, flexibility and strength but for a judgemental eye, I would be considered the healthier one. Looking a certain way doesn't mean the person is the peak of health, there are all kinds of shapes and sizes that function in different ways. I do think carrying around too much extra weight is harmful to the body but so is a lot of other things. Yet I always feel disappointed and angry when I hear that a plus size, body positive influencer is receiving hate because they are losing weight or working out. Judging someone's body for whatever change it's experiencing shouldn't be considered positivity imo.