Except that thin people aren't systemically, socially, politically, medically and economically oppressed by the fatphobic society we live in.
You're not wrong but the funny corollary to using weight as a proxy for health (as though, as I always say, health were a single thing and not a way to describe how the systems of the body are working together) is that thin people have trouble getting their chronic issues diagnosed because they present as healthy due to their thinness.