A 1-1 draw I think is good enough for AC Milan because the opponents they face are quite strong, especially since this match is an away match, where the percentage is a little difficult to steal full points from the host. AC Milan game was also very good in this match, AC Milan was able to shoot 18 shots and 7 of them were on target, this is a pretty good statistic.
I don't think it is good enough, AC Milan should win the match if we consider the stats. 18 shot attempts, 7 shots on target, ideally they can score more than 2 goals. AC Milan coach needs to evaluate the ability of Milan strikers in finishing or in executing the goal opportunities. If their strikers can play effectively or more accurate in scoring, AC Milan should win this match. However, getting a draw result isn't too bad, at least they can get 1 point. But Milan must avoid getting a draw again in the next match to keep their position on the top standings.
I think the same as you that Milan should have won this match, but Atalanta defense can't be considered weak, sharing points is not bad either. Unfortunately Junior Messias shoot the ball in the 23rd minute sideways, if it make a in score maybe we can see Milan won, even though the breakthrough at that time had made me get up from my seat.