While I agree that it’s healthier not to rely on other people’s approval, I think you’re missing the point a little bit. The reason I don’t grok the body positive movement is because fat acceptance is more about accessibility and equity. It’s about making the world a place where people like me don’t have to wonder if there will be a chair we can sit in when we go out with our friends. It’s about ending stigma about larger bodies.
I also disagree with the idea that we shouldn’t be posting our bodies on Instagram (or wherever). Seeing bodies like mine is a huge thing that has helped me to feel more normal, and that’s important. Sure, teaching people that their bodies define them is one facet. But another facet is that it’s okay to like your body, and that sharing yourself when you feel good about yourself is also okay. When I post a picture of my body, I’m not looking for validation. I’m connecting with other people through common feelings.