Look at him screwing on the end bell of the motor at 222 seconds in. [1] That's a big diameter thread, and the close-ups of it show a very clean thread. You can see the marks of the 3D printing layers on the outer motor casing, but not on the thread. I suspect that thread was turned on a lathe.

[1] https://youtu.be/NFvMC3l3fGY?t=222

I've printed finer threads on my cheap 3D printer, although it took a few tries to get them the right size.

It's possible that he used acetone vapour to smooth out the surface of the threads.

that would be a huge tap. the motor looks to be about 4-5" wide. you'd need a $500+ tap for that, no?

Probably not tapped, but single-pointed on a lathe or thread milled: https://en.wikipedia.org/wiki/Threading_(manufacturing)#Sing...