The copyright office claimed works created by a non-human aren't copyrightable at all when they refused Slater, but that was never challenged or decided in court. It's not a slam dunk, since the human had to do something to set up the situation and he did it specifically to maximize the chance of the camera recording a monkey selfie.

If I set up a rube goldberg machine to snap the photo when the wind blows hard enough, how far removed from the final step do I have to get before it's not me owning the result anymore? That's the essence of the case, had it gone to court, probably the essence here too.

My guess is the creativity needed for the prompt would make the output at least a jointly derived work regardless of any assignment disclaimers--pretty sure you can't casually transfer copyright ownership outside a work for hire agreement, only grant licenses--but IANAL and that's just a guess.