The title is a lie if you actually read the case. AI work can be copyrighted if there is enough human input into the work. This case was just about trying to get the AI to claim copyright (which it cannot do as it is not human) and transferring that right to the owner of the AI.
As for cases where the AI work is not under copyright then I believe it would be consider public domain? And thus we already have rules for how that works can be used as part of a greater works.