I don't have an opinion to offer here other than the intrinsic limitations of studies that depend on twins raised apart (that there aren't many of them). It's an unusual instance of a stat where the obvious concern with the premise is underappreciated rather than overappreciated. I've got nothing on MZ/DZ controls.

ah I see, you're commenting on the general difficulty, not necessarily saying this study's results are bad due to the limitation. My apologies, I don't think we disagree.