Just watched a ref on TV explaining the definition of interfering with play.
* Blocking the line of site of the goalkeeper.
* Physical contact with an opposition player (whilst not touching the ball)
Rashford did neither of those things. He didn't touch the defenders or the ball, and when Bruno struck the shot, Rashford was behind him, so Ederson had a clear line of sight. Neither of those defenders were able to stop the ball reaching Bruno, as they had moved up to play the offside trap. So regardless of who they were tracking, they weren't getting to that ball before Bruno hit it. Walker actually realised that Rashford hadn't touched it, which is why he is desperately trying to block the shot.