The full report for those interested.
https://official.nba.com/l2m/L2MReport.html?gameId=0022300159
Cool, cool. The second loss in a row where reffing incompetence disadvantaged the Celtics in the final two minutes on a missed non-call.
You're ignoring the incorrect call of the shooting foul by Maxey on KP. If KP isn't given that free throw, the score never gets down to a 3 point lead. So KP's missed 3 wouldn't have been to tie the game and Embiid stepping out of bound with a few seconds left would have been meaningless.
As for incompetence of the refs, the report reviewed 17 calls/non-calls in the last 2 minutes and 15 were correct. I'd say that is pretty good competence given how hard it is to referee an NBA game.