I admit Qc5 was not the best move. He should have played Qh5 instaed of wasting a tempo. Now, it being hard to believe that a 2000 rated player would make such a move is not that inconceivable. In fact I have seen Gm's miss mates in one.That wasn't even a mistake, it was an inaccuracy. Even the top players make inaacuracies now and again. By the way, it was Correspondence Chess.
Qh5 immediately loses the Queen, 5.Qxh5
Mistake/inaccuracy depends on your perspective. I've seen Tarrasch describe the most innocent looking moves as fatal errors.
GMs tend to talk like that..... but if you think about it..... at their level, it's true.
The size of a mistake is relative to the player. I edited my comment with the game:
Fantastic kingside attack.