Dean says his mistake was that he didn't think the hair pulling was violent conduct, so he didn't ask referee Taylor to look at the video monitor. In other words, Dean decided that a red card was not warranted. Dean decided to become the referee of that incident in the game, even though he was only the VAR.
Typical of Dean this. His is the only opinion that matters in real time. He really isn't fit to be a VAR, because he will take decisions away from the only person that should be making those decisions - the referee.
In this case, he should have realised that Taylor might have had a different opinion to Dean, and asked Taylor to look at the incident to decide whether or not violent conduct had occurred. Taylor would then have agreed with Dean, or gone against Dean and issued a red card.
This still doesn't absolve Taylor of his incompetence in not seeing the incident in the first place, as it was directly in front of him.