If he had blown his whistle after the foul on Haaland, then watched Haaland put Grealish through, and at that point shout "Play on, advantage", and indicating with his arms, would this have been a correct decision? Obviously not.
But he did a very similar thing. He played advantage, which was the decision for the foul situation. That decision is then final for that act, and it is not possible on footballing law to change that decision.
It wasn't a split second mistake, it was a conscious choice to change a decision already made. There was no footballing reason to change the decision of advantage that had already been made.