Denial/wishful thinking are extremely powerful.
That said, body positivity shouldn't be thought of as having anything to say about objective physical health: there is zero reason a person who is obese should necessarily feel bad about themselves. Shame is a useless or detrimental approach to changing behavior; shame is oftentimes the reason people engage in unhealthy behaviors like overeating in the first place. Being overweight is an obvious health danger, but it does not make you a bad person.
Somewhere in the idiot echo chamber of social media the idea of a healthy sense of self worth got conflated with a healthy body.