It is, you’re right. But this study isn’t supposed to address all outcomes, so it’s stupid to complain about it. Just like studies relating BMI to physical health usually don’t incorporate the mental health impact into their analysis. Because it wasn’t in scope for the study.

If this was a dissertation on all health affects of BMI and it didn’t contain information on the physical aspects that would be an issue. But it isn’t, and the people complaining about it are making an issue out of nothing.