However, simply asserting that something explains the results while not addressing the ways that the study actually look at that factor is not a valuable contribution to the conversation.
You are correct that self-reporting adds error to the BMI and the optional self reporting may have narrowed the range of BMI reports than can be analyzed. Both of these would reduce the size of the predictive effectivenes. It does seem like an area where a study better designed to control for BMI would increase our understanding. However the relative effect size vs lower face shape indicates to me that it is pretty unlikely that BMI fully explains model predictive ability.
However, simply asserting that something explains the results while not addressing the ways that the study actually look at that factor is not a valuable contribution to the conversation.
You are correct that self-reporting adds error to the BMI and the optional self reporting may have narrowed the range of BMI reports than can be analyzed. Both of these would reduce the size of the predictive effectivenes. It does seem like an area where a study better designed to control for BMI would increase our understanding. However the relative effect size vs lower face shape indicates to me that it is pretty unlikely that BMI fully explains model predictive ability.