We know with absolute certainty that the referee did not see a handball because there wasn't one.
So either he made it up or imagined it. My point is, this wasn't an in-play decision, the goal had been scored and the ball was out of play when he made his "on-field" decision.
The correct approach even if he thought he saw a handball, was to give the goal and then review it as they do in virtually every game. It can then be overturned if there is a clear handball. By doing what he did, when there was absolutely no need to, tells me that he wanted it disallowed.