I'd say he's more proof that a sufficiantly strong popular movement can force the Democratic party to acquiesce to their will. The party serves more as a limiter on socialist politics.

Remember that to this day the top Democrat, who lives like 5 miles away from Mamdani in the same city, refuses to support him.

Remember that they tried to resurrect Cuomo to run as an independent against him when he secured the Democratic nomination.

Socialists can get stuff done. It's just that due to the nature of the US political system they have to call themselves Democrats and submit to an anti-socialist Democrat gauntlet.