Over time, the laws of the game have moved in a direction where to be penalised for being in a offside position without touching the ball, you have to one of four quite specific things. The aim was to be consistent with the application.
Rashford doesn't exactly do those four things, but you can make an argument that he does. There's definitely an element of interpretation needed, there's some subjectivity about it. On the day, the officials didn't feel he had interfered in the way it is defined in the law. That was supportable.
If that same circumstance happened again, you'd probably see a different outcome now based on all we've taken on board since then.