I think the poster is looking at it from the other way: purely machine-generated content is not generally copryrightable, even if it can violate copyright. So it's more a question of can a coplyleft license like GPL actually protect something that's original but primarily LLM generated? Should it do so?

(From what I understand, the amount of human input that's required to make the result copyrightable can be pretty small, perhaps even as little as selecting from multiple options. But this is likely to be quite a gray area.)