I do not recall anyone saying he wasn't a citizen...just that he is not a natural-born citizen. My wife is a citizen, but not a natural-born citizen.
Since he was born to an American woman, he was an American from the moment of his birth. That makes him a "natural born American". There is no requirement that he born on US soil, which he was in any case, just that he be a natural born American and not a naturalized immigrant.