What utter horse crap. That is not how dual citizenship works.
But will anyone else challenge his statement?
Actually, that's exactly how it works. When two different countries claim the same person as a citizen, that person is a dual citizen.
Which brings us to Millhiser's point, which is entirely valid. If the birther claim that dual citizenship disqualifies a person from being president is true, than any rogue state, say Iran, could effectively veto a US presidential election by simply declaring the winner to be a citizen of Iran.
Now I know Millhiser is a leftist kook and all that, but that doesn't invalidate his point. Broken clocks and all that.