I didn’t say that. In fact I didn’t say anything about mixed heritage. Perhaps you have me confused with someone else.
I did not say anything about mixed heritage, I said mixed parentage. In other words, if someone has a U.S. citizen mother and, say, a Canadian citizen father, your logic would hold that the person is not a natural-born U.S. citizen (because of the Canadian father), and that the person is not a natural-born Canadian citizen (because of the American mother).