It's just a dietary heuristic, why would it have to explain everything? If you want that, just go and look at the literature on overweight and obesity or, say, substitution of animal protein for plant protein. It's all there.
Under that usage, the fact that the rule doesn’t provide fine-grained advice doesn’t disqualify it from being a heuristic. Eating mostly plants is a rule used in decision making when considering what to eat.
> The extreme is that it is advising people to seek treatment if they suffer from pica or bulimia.
How is that entailed?