This argument falls apart when you consider digital photographers. They didn’t create the image - a machine and software did. Would you make the same argument that a photographer’s only copyright is the input to the camera settings and the color-grading software?
Proof of process is totally reasonable, since all art should have a clear provenance, and I’m sure the original prompt & seed could prove his ownership.
This goes beyond your point, but the argument that AI Art can’t be copyrighted because it was trained on copyrighted works is a little hackneyed. Every human artist is trained on copyrighted works, reproduces that art during school, and often incorporates referential elements into their own art.