I think Mamdani very genuinely wants to help people rather than just enrich himself and do corporate bidding, which is more important than doing the most leftist option available at any given time. If we put good, selfless people into positions of power while removing and condemning the selfish and corrupt, we will be better off as a society.
Personally, I'd rather have a caring and compassionate slightly left politician than a hard leftist that only focuses on politics and playing the game.