He has a metronome.
This was how you did it 40 years ago with an 8mm camera, home tape recorder, and a metronome in your rec room. Good enough to be one of the few home movies to be on the Library of Congress National Registry of Films (the impressive stuff starts at 4:30)
Fat lot of good it did him.
That’s the one option I don’t think he did. Not only did I not hear it, but at no point does he seem sped up or slowed down. Plus, if you were going to audio edit it, there are a lot more little touch ups you could do. It could even be mixed a bit better.
Aha! He has answered a user comment on his video, that says