It's one of the few resources available for historical city populations, and its recent estimates more or less line up with what citypopulation.de reports, but if they have sources, they're not telling.
If it's not reliable, does anyone know any better historical city population data resources?
It seems to differ slightly from this page:
https://en.wikipedia.org/wiki/New_York_metropolitan_area
Probably because the MSA boundaries have shifted over time.