Using plagiarism rules seems both fair and appropriate (given the source of the AI's content).

A human reviewing an author's writing and then passing it off as their own isn't okay. If they make substantive edits such that it's essentially new work, then it's okay. Discussing with the author then writing their own versions is okay.

Really though what I'm after is a mass protection so consumers can understand that images they're seeing or language they're reading or hearing isn't from a primary (ie human) source. Sort of like the "actor portrayal" or "this image has been photoshopped" requirements some countries have.