I'm not asserting that human and AI outputs are generally indistinguishable. I'm stipulating that they are in this particular hypothetical to isolate the question of provenance.

Even if AI were entirely incapable of creativity, why would incorporating its output diminish the creativity of the human artist?

What about stock samples, preset drum-machine patterns, or clichéd drum parts played by a human drummer? Do these diminish the artist's creativity, and if so, why?