You're assuming that a monkey that eats 20% less than a monkey that's given the choice to eat as much as it wants necessarily isn't underfed? Maybe. But without some other research that I've missed, that really begs the question.

I think anyone living in a western country has enough data to realize that people eating a western diet tend to be overfed, so the comparison between westerners and Tibetan monks has little to do with monkeys.