At least one group of cannibals in the module
eats zombies (p. 196-197).
I would say that the answer is ambiguous, perhaps intentionally so. We're talking about a module that tells you that dinosaur races happen "weekly" in a world that uses a "tenday" instead of a week.
Or give you detailed instructions about travelling during tropical storms, gathering rainwater, and dealing with extreme heat; but no explicit guidance on how often extreme weather events occur.
So, I'd say that it is intentionally left to the DM to decide
- if I want to include cannibals
- how horrifying do I want them to be
- how many taboo things do they need to be willing to eat to get the
effect I desire?
On a subjective note, I'd infer "willing to eat any of the party members" if you're playing it straight. If you are looking to inject more of the humor that gave you goblin stacks, "Nanny Poopoo," "Biff Longsword," and zombie-belching zombie dinosaurs, a tribe of ethical cannibals that only eats zombies could be a fun bit. (Now that I think of it, I might do that in my campaign!)