The citation about citizenship means "U.S. citizenship", not Chinese. It is the basis that allows the U.S. to tax residents of the U.S, not just its citizens.
I still believe that his Chinese citizenship should take precedence over the fact he got the visa from Singapore. However, that is a technical question which may be better presented to someone with a lot more experience in this field than me. For this reason, I suggest you post your question on the forum at
www.thetaxguy.com.
The forum moderator, Dr. Gary Carter, is a CPA who possesses a doctorate in taxation who specializes in taxes for international student and foreign guests to the U.S. He has been most helpful in the past, and is probably the best person to ask about your situation.