If they can prove that all of the art used to train an AI model is their own art, and treated the output as derivative work, then they should be able to enforce copyright the output of that specific AI. Using public domain works might be possible, although there would need to be some kind of significant portion that is theirs so the AI isn't used to copyright public works that were simply fed through the AI.
Note that I didn't say who created the AI model, as the AI is a tool and not the creator of the work. The problem with the current implementations of AI and copyright are that the models are trained on a mix of copyrighted and public domain works so there is no way to know who to identify the derivative work that comes out of the AI.
Personal opinion as I see AI as something like Photoshop that outputs something with a change based on an algorithm, with the AI just being a far more complex algorithm.
If they can prove that all of the art used to train an AI model is their own art, and treated the output as derivative work, then they should be able to enforce copyright the output of that specific AI. Using public domain works might be possible, although there would need to be some kind of significant portion that is theirs so the AI isn't used to copyright public works that were simply fed through the AI.
Note that I didn't say who created the AI model, as the AI is a tool and not the creator of the work. The problem with the current implementations of AI and copyright are that the models are trained on a mix of copyrighted and public domain works so there is no way to know who to identify the derivative work that comes out of the AI.
Personal opinion as I see AI as something like Photoshop that outputs something with a change based on an algorithm, with the AI just being a far more complex algorithm.