Simply put, if you learn how to do said checkmate, and you have a king/rook and the opponent has nothing but a king, you will always win. There's nothing he can do, but over an entire game, said person always has a chance.
Try playing Stockfish or Leela or AlphaZero. There is NO CHANCE for the opponent if you make no mistakes.
My point was, that it is possible to make a mistake in a Q+K endgame and stalemate it. I know it is an absolutely certain win, if you do not blunder stalemate, but it happens. One has to be very aware of that chance of stalemate at all times in the endgame when the opponent only has the king and/or pawns that can't move.
My point was not that it is not a certain win if one does not make a mistake, but human beings make mistakes. That is why Stockfish for example will crush you, me and Magnus Carlsen every time. EVERY TIME.
'do you hear that, mr. anderson?
that is the sound of... inevitability:
it is the sound of your death'.