Thumb Position Against Back Of Neck

Hey guys,

I recently got a new guitar instructor who is trying to teach me to improve my shredding skills (I don't think he can shred tbh).
He's been telling me I need to keep my thumb away from the top of the neck of the guitar (completely out of sight, and very close to the bottom).

I've never played like this before, it's uncomfortable and feels unnatural to me. It can even hurt at times. I've been watching and it seems like some of the fastest players I could think of play like me: thumb poking above the neck; Marty Friedman, Yngwie Malmsteen, Alex Laiho, Steve Vai, Jeff Loomis, Nick Johnston, etc...
Eric Johnson seems to play with his thumb an extreme amount above the neck.

I was wondering what you guys all thought about it?
My teacher is making it sound like a life-and-death situation here but it works fine for a lot of amazing guitar players.