The people who turned up were the 20-somethings who are politically-minded

When voting turnout exceeds expected numbers, we call those additional voters 'low-propensity'. It doesn't matter if it's a national election or a local one - when turnout blows out expectations, that's a high-enthusiasm election. Trying to describe those low-propensity voters as 'politically-minded' seems intentionally misleading, since I can only assume that's based on the fact that they turned out when they were expected not to (i.e. they turned out because they responded to a typically low-turnout election, thus they must be 'politically-minded').

Setting aside the circular definition - any time a candidate is able to turn out more voters than expected, that's a definitionally good candidate by any electoral standard. The question isn't really 'who would non-voters have voted for if it were a national election?', but, 'does this election translate to a national voter base?'. And while that's not something you can easily generalize, Mamdani did run on policies that are resoundingly popular in all 50 states. There's very little reason he wouldn't have performed better-than-average on a national stage given what we know for certain.

All this to say: anyone trying to downplay the significance of an Indian-American, Muslim, Democratic Socialist sweeping an election against one of the most famous political dynasty names in the US, where corporate media across the entire political spectrum were united against him, and where opposition spent tens (if not hundreds) of millions of dollar more than him - and in of all places the financial capital of the world and in a city that was the sight of the most famous terrorist attack conducted by Arab Muslims in the western world - is absolutely coping. That kind of candidate winning in a place like New York would have been inconceivable since at least 2001.

You can deny it as a significant moment of socialist achievement if you want, but you'd be fooling only yourself.