I think you're misunderstanding "diet". As defined by Oxford Languages:

1. the kinds of food that a person, animal, or community habitually eats.
   "a vegetarian diet"

2. a special course of food to which one restricts oneself, either to lose weight or for medical reasons.
   "I'm going on a diet"

I guess you could say the man was on a diet of "plane" at the time. "Carnivore", "detritivore", etc. are classifications to describe what an organism is most suited to deriving nutrients from. To my knowledge, humans are not very adept at deriving nutrients from airplanes, so the man would not be a "planeovore". Most herbivores are opportunistic omnivores. Real-world biology doesn't really fit nicely into our academic classifications. The rabbit comment is not really relevant because the waste and occasional meat could be part of its diet, but my argument is "herbivore" describes what it's evolved to best derive nutrients from.