Maybe it's this? Peter Attia https://www.youtube.com/watch?v=U3oI104STzs especially from 8:45 when he talks of the people who stay thin (which is worse for them, if they actually are insulin resistant)

Unfortunately, no. It was a theory put forth by a researcher on a podcast.

It makes complete sense to me, but I don't know how you'd even design an experiment for this. Maybe continues glucose monitoring along with identical diets for people of various body fat percentages?