The simplistic American system of citizenship is not universal. Being born in a country does not always make one automatically a citizen of that country. So too does being born in country A not negate one's automatic citizenship in country B.
From the perspective of China, that a person was born in the US is immaterial. If they are born to Chinese parents then they are Chinese, especially when now standing inside China. This is why one should always talk to a lawyer before visiting "the old country" with kids born elsewhere.
https://en.wikipedia.org/wiki/Chinese_nationality_law
Bigger horror story: First generation Iranian-Canadian family fly to Iran to introduce canadian-born grandchildren to Iranian-born grandparents. At airport for flight home. 19 years old? Male? born to Iranian parents? standing inside Iran? Have you done your military service?
Whether one can "disavow" a citizenship is a big question. Citizenship is not a hat. You cannot just move to Canada, marry a Canadian, and suddenly expect not to owe US taxes ... or not be drafted into a US war.