You're right, it should probably be more comprehensive that just "AI Artwork" so we will also be including voice overs and writing in this. If somehow you and every other person submitting a game could come up with a way to prove that their AI artwork, voice overs, and writing was not trained using/based off of other people's work, then we will happily change the rules.
However, we are not here to argue the ethics of AI materials. We've made a decision we are comfortable with, and we will be sticking with it.