He did not get into Columbia.
The NYT granted the source of this hacked information anonymity, despite his name being well-known, to cover up the fact that they used a Nazi eugenicist freak as a source of hacked information. The anonymity (pseudonymity, precisely) was not to protect the source, but to try to protect the rep of the Times.
They even tried to determine whether Mamdani's family ever intermarried with indigenous Ugandans, which somehow was important to this story of how a 17-year-old correctly checked the boxes on his form.
There are 3 bylines on this story.