I think your thought experiment is begging the question by asserting the outputs are indistinguishable.
The poster above is complaining that the actual outputs are qualitatively "not creative". That they embody a statistical norm, while creative work has more unique variance.
These two viewpoints cannot be bridged by a flourish that ignores the fundamental difference of opinion...
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?