In the interest of "objective facts and truth":

That's not the actual slogan, or what it means. It's about pursuing health and measuring health by metrics other than and/or in addition to weight, not a claim about what constitutes a "healthy weight" per se. There are some considerations about the risks of weight-cycling, individual histories of eating disorders (which may motivate this approach), and empirical research on the long-term prospects of sustained weight loss, but none of those things are some kind of science denialism.

Even the first few sentences of the Wikipedia page will help clarify the actual claims directly associated with that movement: https://en.wikipedia.org/wiki/Health_at_Every_Size

But this sentence from the middle of it summarizes the issue succinctly:

> The HAES principles do not propose that people are automatically healthy at any size, but rather proposes that people should seek to adopt healthy behaviors regardless of their body weight.

Fwiw I'm not myself an activist in that movement or deeply opposed to the idea of health-motivated weight loss; in fact I'm currently trying to (and mostly succeeding in!) losing weight for health-related reasons.