Did you watch the match? If you watched the match, then you will see that Arsenal struggled in the match, and Arsenal didn’t play anything special in the match. I will say Arsenal were just lucky in the match, Manchester United dominated the match, they created lots of chances, but the issue is that Manchester United didn’t have a player that will convert those chances which they had into goal, and that was what really affected them.
That's not an excuse. If you dominate the match and still can't score, that means you're not good enough. Arsenal also dominated the last three seasons but always stayed one step behind the leader. If they had a proper striker last year, they would've been champions. They've filled that gap this season. Manchester United also need to address their weaknesses. Otherwise they can't compete for the title. If you don't get the 3 points, it doesn't matter much what you did during the game.