He was born a US citizen though. The requirement isn't that you are born on US soil, but that you are born a US citizen. John McCain was born in Panama.
That said, although legal scholars can have how many angels can dance on the head of a pin debates about this, the most reasonable interpretation is probably that they're US citizens/eligible for US citizenship from birth. If push ever came to shove on this question, one would hope the courts would side with the most liberal interpretation.