Wyobraź tu sobie napis~ - ...just made an illegal move in chess, so it doesn't make sense that he could say checkmate, he might possibly say check but the use of the term here is wrong. The only way this could be a legal move is if this were blitz chess, also known as "Fast Chess". However, in this scene, it is never declared they are playing "Fast Chess", and neither of the players are using clocks to time their turns in the game.