I personally feel the mere fact that it was fair use of mostly copyrighted images it's fairly self evident that anything produced from it should not be copyrightable, UNLESS the origin of the art used to train the model is 100% owned by the "artist". This could either via licensing for that purpose or they own the copyright to, that right not extendable to corporations, as a corporation can't be an artist. It doesn't matter how complex the prompt or series of prompts are, the key here is that the "artist" either owns the training material or licensed it through the proper chain of licensors.