Saw highlights:
1stly, Looks to me like Maguie moves his boot in a manner in which he's trying to block the ball, just looked again and he's definitely trying to block it. So couldn't be offside because "A player in an offside position receiving the ball from an opponent, who deliberately plays the ball (except from a deliberate save), is not considered to have gained an advantage. "
2ndly, "“interfering with an opponent” means 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 " wouldn't apply anyway as De Gea can see the ball perfectly well.
3rdly, There was also a foul on Sigurdson that left him in that position in the first place, two foot up sliding lunge in to him after playing the ball which is a stone-wall penalty.
Not as bad as the no red card for what was basically a criminal stamp by Lo Celso in the Chelsea game last week though.
Pretty standard premier league VAR decision this season all up...