Not sure what you're asking. We have one code that defines natural-born citizen/citizen at birth. Why would we need another?
It's your question not mine. Your question is where is the code for NBC and I'm saying there is not one needed, as you acknowledged I'm not questioning whether they are, but whether they alone are.
So you acknowledge that they are but can't accept that there is not a law needed for them and because there is a law for all other types of citizenship we need one for the only obvious type.