I think this relates to the odd curiosity about his education in the U.S. on a student visa from Yemen. He may have been born in the U.S., but if he was a U.S. citizen there would have been no need for him to obtain a visa to come back here as a young adult. From what I've read on this subject, I suspect his parents actually went through the trouble of establishing Yemeni citizenship for him as a child, which would have required them to strip him of his U.S. citizenship under Yemen law (again, this is partly speculation on my part but does explain that oddity of his student visa).
Which that whole idea is turned on its head when you consider the adoption of 0bama by his father in Indonesia. Indonesia doesn't recognize dual citizenship either. LOL. The adoption would have stripped him of his citizenship.