If we're gonna be technical it's Bouchard's man.
And McDavid can't just teleport to the front of the net, watch the play again, he is forced to go around the Avs player behind the net, you can't just go through a player. That gave the player that was Bouchard's man a split second to jump out in front of the net uncovered and Campbell then gives him the opening to put the puck, McDavid did try to recover but you can't just make up 3 strides from out of your ass in a millisecond, not even Connor McDavid can do that.
Broberg probably needed to be aware that Bouchard was doing nothing and he needs to slide over, he was the closest Oiler player in that situation to the guy who scored to goal. Both of the D are stuck covering the left side of the net.
Watching it even again, McDavid could've basically ended Byram there if he went for the hit instead of skating around him.