Chelsea must consider themselves lucky for not losing this game actually. Because comparing the general performance of each team, Manchester United were clearly the better side. They found many more opportunities to score in this game but Chelsea were still the first team to score anyway. The last minutes were pretty exciting though. Manchester United were in big trouble after conceding the goal in the last minutes but Casemiro saved the team one point by scoring the equalizer in the last moments.
Erik ten Hag must be disappointed with the score after all that effort but I believe it is still nice to steal some point from a big team in an away game.
Nothing like lucky because the match changed to be against Manchester united after Manchester united suppressed Chelsea for like eighteen (18) minutes constantly half field. So if not that time and the change Chelsea made, and the match balance to come against Manchester united, Manchester united should be the one that supposed to be lucky because the got their goal at the die minutes, and if not the mistake of Chelsea goalkeeper the goal won't have entered.
You are right, Manchester United indeed dominated the match but they were lucky because Casemiro scored in the 94th minute
Chelsea should have managed to get 3 points but all disappeared in the final minutes

but yeah 1-1 is fair enough for both teams