I agree, for even if you re-watch the match it is clear that Arsenal were superior to Manchester United at many points, so the fans' outrage is nothing more than frustration at the defeat. After all, even assuming that their goal would have counted, it wouldn't change anything, for the score is 3-1
Yes, even with all this performance Arsenal had yesterday, the Manchester United fans can still agree that they lost the match because of their over-hyping. Although Manchester United weren't playing the ball with seriousness, if you check very well, Arsenal just didn't want to lose or draw the game as Manchester United did. That's why Arsenal played with full force and all the rest.
For Arsenal to beat Manchester United by 2 goals, that means they might beat Manchester City also if they meet next and with this performance they can go forward in the league.