I'd guess there's like an 80%+ chance Mandani's parents and/or school counselor encouraged him to check the box for African American. But I think the much bigger deal is how this story got to the New York Times in the first place, how they reported it, and what that reveals about how they operate.
It's not that long ago that the Times refused to published hacked and leaked materials that were much more newsworthy: the Trump campaign's oppo research file on Vance. But here, a freelance medical reporter (whose main beat appears to be anti-trans shit) was given hacked college student application records by Jordan Lasker, who is friends with Richard Hanania, is a fairly well known white supremacist, eugenicist, and peddler of racist pseudoscience, and is grad school dropout who committed academic misconduct so severe he got a tenured co-author of a paper he worked on fired. In short, he's a fucking nazi. Also, this isn't as big of a deal, but his twitter handle "cremieux" is a cum joke and his reddit handle is trannyporn0.
Now, depending on the information and the story, maybe the fact that the source is a nazi doesn't keep you from reporting it. But a real journalistic outfit would be pretty fucking careful. They'd want to make sure they weren't being used to advance the nazi's agenda and they would want to be honest with their audience about who their source was. Did the Times do any of that? No. They were happy to tell exactly the story the nazi wanted them to tell, they gave the nazi anonymity, and even described him as an "academic" so their audience would think the source was something other than a dipshit, gutter racist little nazi freak. And published this on the same day SCOTUS just gave the OK for Trump to illegally render people to third-world torture prisons and the GOP passed their bill destroying Medicaid and giving ICE a bigger budget than all but two militaries on the planet get.
The Times' ownership has reduced it to a shitty nazi propaganda rag. And the real cherry on top of this story?
They did all this so their source could try to make some money gambling on the effect he thought the story would have on the prediction markets, but it had none.