Body positivity has roots in the fat liberation/acceptance movement. It was initially meant for fat people. It's just totally ahistorical to say "but it's meant for everyone" when that's literally not why it was created. https://metro.co.uk/2019/07/01/the-body-positivity-movement-is-not-for-slim-bodies-already-accepted-by-society-10081795/

The fat acceptance movement is about recognizing that fat people face oppression and discrimination and working towards a world where that oppression doesn't exist. How can you centre yourself in a movement to end a form of oppression that you don't face?

That doesn't mean that thin people aren't allowed to feel "positive" about their bodies, but it does mean that they shouldn't hijack the body positive movement.

Having "insecurities" is not the same thing as being oppressed.

Being bullied is also horrible but it's not the same thing as being oppressed.

If thin people feel that they need a movement because the fat acceptance movement doesn't "include" them, then they should make one. They can take some tips from the people who started the Straight Pride March.