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.
This is going to shock you, but they aren’t related. You can be obese and be healthy. The science shows that diet and exercise is what is correlated to health, not fat percentage. And for elderly people, life expectancy is higher in overweight groups. The assumption that you know someone’s health by looking at their body is harmful and unscientific.