VAR are authorised to intervene if they believe there has been a clear mistake by the ref. It's usually when they believe that the ref hasn't seen something. In this case it's impossible that he didn't see that Rashford was miles offside, that he ran towards the ball until he almost got to it, before letting Fernandes hit it.
He chose to interpret that as not interfering. In effect, he is interpreting that if you don't actually touch the ball, you're not interfering. You are not having any decisive effect on the behaviour of the players around you. The defenders, the keeper.
Enough has been said about that. We all saw what happened. I have nothing to add.