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.
It's liberal identity politics, seeing all social issues through the lens of "Identity X is oppressed by majority Y, and we just combat this by proclaiming the virtue of identity X". And the liberal view of oppression basically consists of others having negative opinions of the to the given identity and acting on them, not a structural problem that exists regardless of the motivation of actors within the system. The way they talk about the pay gap, which does exist, but not quite in the form liberals often claim it does, is a perfect example of this.
So while socialists might see obesity and the issues surrounding it as extensions of artificial scarcity, capitalism making food as cheap as possible in the name of profit, poor people having less free time, and a Healthcare system that's based on making money instead of promoting health, liberals can't see past "society is mean to fat people", or if they can, then all their analysis stems from that point.
There's no material solution offered in this massive post, just tolerance and acceptance, or as Cushbomb likes to put it, fighting in the realm of the symbolic. Because being a walking joke does feel shitty, and for fat people, at least not being treated as a disgusting failure of a person would probably be nice. But that's the limit of the liberal imagination: being nicer.