No, it is not.
Cheating is using non-standard methods to create an advantage. He is using pretty standard method -- sitting straight and doing nothing. Tanking is not even an exploit, because it would be using bugs or doing something similar that game designers didn't intent a player to do. Not promoting is an obvious thing definitely known to the game designers. You can call tanking whatever you want, but it is not a cheat nor exploit. It is an ugly feature of this game.