It's entirely based on the interpretation of law 11

Here's the relevant bit:

interfering with an opponent by:

preventing an opponent from playing or being able to play the ball by clearly obstructing the opponent’s line of vision or

challenging an opponent for the ball or

clearly attempting to play a ball which is close when this action impacts on an opponent or

making an obvious action which clearly impacts on the ability of an opponent to play the ball

Now, it really comes down to the last 2 bullet points. You can take a very conservative view of law 11 and say that since Rashford didn't contact an opponent, didn't attempt to play the ball and didn't directly interfere with the run of a defender, it isn't an offense. However, I think that's contrary to the spirit of the law.

He made a long run, defenders had to track him and he didn't pull out of the play until the last moment, while in traffic but not necessarily interfering with play. I would still call it, however, being actively involved. I know they don't use that term anymore, but it's a good way of framing it.

Frankly, I'd have given it and had the AR put his flag up when Rashford got close to the ball - and the referee immediately blown the whistle - nobody would have complained. But because they held off, as they are told to, VAR decided that they would over-analyze instead of using common sense. Which seems to happen often in England.

I've said this before, but it's worth repeating: considering they have the most influence on making the laws of the game, they often suck at enforcing and interpreting them in England.