How can you not be interfering with play when you are sitting upright, a couple of yards in front of the goal - between the goalkeeper and the ball, and then have to actually move out of the way of the ball, because you are directly in the way of the ball travelling towards the goal? If he hadn't moved it wouldn't have gone in, so he influenced the goal, from an off side position. Also, when Dom took the shot, he is definitely in De Gea's line of vision. It didn't make any difference, because of the deflection, but he was definitely in exactly the wrong place.
Would you have the same opinion if this happened at the other end? Or if it wasn't in the dying seconds of the game?
The only defence I can make for him is that he made a judgement call to sit still - in order to not be interfering with play. Unfortunately for him, the ball went straight at him. This meant he had to move out of the way - which brought him back into play.