Is there any evidence that Apple has actually made this "threat"? I'm not seeing anything other than what Patreon has claimed (and it seems that they are only recently going to begin to allow iOS purchases, which might mean they are bringing this upon themselves).

I am suspicious, because the specific change to per-creation billing is overwhelmingly positive for Patreon (and, as you pointed out, not for its users), from a business economics perspective (assuming they don't lose too many users over this). It also seems odd for Apple to press that point specifically.