It isn't a dietary heuristic, because there's little advice provided. The extreme is that it is advising people to seek treatment if they suffer from pica or bulimia.
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?