yeah, but technically as long as he doesn't become "airborne" and jump towards the plate, it is legal. I know the legal pitching motion rules in the MLB rule book are something like 5 or 6 pages long, but I don't think it says anything about when exactly the back foot can lose contact with the rubber as part of the pitcher's natural pitching motion.
So as long as he can convince the umpires that he is dragging his back foot and not hopping (
which is B.S., to slide on his back foot like that while his drive leg is still off the ground is impossible, he has to be losing contact with the ground at some point), they'll let him keep doing it.
Should be illegal based on the physics of what he is doing, but isn't illegal based on what the umpires can and can-not visually confirm.