Harmful for the same reason as the 'healthy at any size' campaign, and anti progressive because if they claim health at every size, then they deny there's a huge malnutrition problem in America. It would be solving the problem of food deserts and obesity by just saying 'no no these people are actually healthy' instead of making sure they don't live in a fucking food desert.
There was a very deliberate effort to push the idea that HAES meant being overweight was healthy by reactionaries, to the point where the misinterpretation and the overblown reaction to that actually happened overshadowed the actual ideals of the modern movement. Anyone on reddit a few years back might recall /r/fatpeoplehate and /r/fatpeoplestories. From https://www.nationaleatingdisorders.org/size-diversity-health-every-size:
"The basic premise of health at every size, as written in [Lindo] Bacon’s Book, Health at Every Size: The surprising truth about your weight, is that 'Health at Every Size'(HAES) acknowledges that well-being and healthy habits are more important than any number on the scale.
The way informationated is disseminated these days is troubling. It's not like there was ever some golden age of truth, but social media and the internet in general has really amplified the way misinformation spreads.