Nice quiz. I’m not sure I’d automatically consider irreligious people to be atheists, though. I did minimal research and I’m no expert on religion in China, but that number seems pretty high to me. From what I’ve read, most Chinese people don’t follow an organized religion, yet many still hold folk or ancestral beliefs rather than being strictly atheist.
From what I've looked at, it seems "atheist" in terms of the Pew Research Center is someone in the middle of "irreligious" and "convinced atheist". Even still, most of the cities on the list would still be in East Asia. But you're absolutely right- they should be more specific on what they define as "atheist"- folk religionists or those who follow a traditional religion aren't automatically atheist.
Sources: citypopulation.de, the Pew Research Center, the Jewish Virtual Library and cartomission.com.
If going by the Pew Research Center's 2014 report, New York City would have ~13 million Christians.