Can confirm it's not a YouTuber.
That's good. But, then as you said it's some kind of hack. Is it possible to identify his actual username in Minecraft? Maybe the anti-cheat can do that but I don't know if the developers has that feature, or if it is even possible.
I did however, search on google about name-protect plugins or something like that and found out that some anticheats has a thing that prevents this type of cheat.
Just thought, would sentinel be able to ban this player with his "fake" name?
But, you will have to confirm me on this. Anyways, hope you figure something out soon! ;)