In “the best played game of chess Hogwarts has seen in many years” Harry is a bishop. Yet he effects the checkmate via moving “three spaces to the left” which is not something in the repertoire of a bishop.
Also, you would think that considering the chess game was supposed to be an obstacle to prevent one from getting the most valuable object in existence, it would be a little better than to fall for a simple ploy like the one Ron engaged in.