I agree the motive is unknown. The options seem to me to be these:
(a) innocent mistake. This requires the referee to have committed three serious errors within about a five second period.
The first is that he allowed a foul to go unpunished in order to see if an advantage accrued, in circumstances where it was pretty unlikely there would be any advantage, because of Haaland's loss of balance, his position and direction of travel on the field and relative lack of blue shirts.
The second is that he signalled an advantage without meaning to. Communication for a referee, including the use of gestures, is something as I understand it they are very hot on. You know when a ref has given a free kick for a handball, and even during the 6-1 we all knew why Clattenberg didn't give the penalty on Micah when he did that stupid fingers-entwined gesture. So this is a second monster error within a second of the first.
The third is that having - on Webb's explanation - decided to wait to see if an advantage accrued, but having made that decision, blew inexplicably early before seeing whether any advantage had in fact accrued. Having decided to wait and see, he didn't wait and see. That is perplexing beyond belief.
(b) Deliberate decision. Having decided to play an advantage he changed his mind after Haaland hit the through ball and Grealish was in the clear.
Insert your own reasons as to why he might have changed his mind. None of them are innocent.
Me? I'm just going on the probabilities. The more you have to invent facts to fit your theory, the less likely it is to be true.