I think OPs point is more about the fact that it’s just a mayorship (albeit in a really powerful city politically) and not some sort of national scale win (although that is also pretty debatable considering the DSA has 3 members of congress and isn’t even a proper political party)
NYC’s mayoral race is typically a good litmus test for what’s going on inside the DNC, largely because the entire city is pretty much the makeup of their coalition. While historically this in no means mean he’s gonna be president someday (he’s not eligible even) it’s a huge indicator of where the democratic voter base is leaning, and typically NYC tends to be much more conservative than this