I don't think that this is a hard rule. They probably look for the same signs that we do - plausible sounding utter gibberish. They just don't want the drop in quality due to that. If an author creates content with AI, but takes their time to edit and improve it, I think that the Gentoo team may give it a pass.
When you write a copyright notice you aught to specify which code is actually copyrighted and which is AI written? Guess you can just include the code and pretend you wrote it, or just omit which part is actually the non-copyrighted AI code.