When I look at the inconsistent barely red glow of the part, I'm thinking he's tempering it almost back to annealed, but not hot enough for heat treat freezing of . Unless more editing occurred, I could see if he put some sort of carboning coating as part of heat treat to surface harden it.
But in the end, you want it to be soft, so a hammer can knock it true. Or put some soft jaws in.
So he gets the body of the small chuck running true, how about that pulley, I didn't see him knock that true.
I just think it's a thing youtube producers do. Some make useless content, then use AI to translate it, and animate different words in the video, with random computer grabbed photos.
Just over the past few months, how many high rated youtuber machinist have put out some version of a burnishing tool using a roller bearing and a ball. And I don't recall any real good surface finish from the ones I watched. As I think the material that is being burnished is critical to it's 'pushability', and not make micro flakes that will gall later.
If I want a ground surface finish, that's the way to do it.