I don't think he is, either. It was a "for sake of argument" post.
There's two avenues of argument about his status, but which one governs depends on where he was actually born. The more subtle one is that Anne Dunham hadn't been around long enough to confer citizenship maternally. The obvious one is that he was born somewhere else, and/or that his primary citizenships were British and then Indonesian.
There is no Question about it, BO’s Father by bo’s own admission was a British/Kenyan Citizen. That is bo’s “Dreams of my Father” story and he is stuck with it.
Now I fully expect that bo being the prevaricator that he is will try to claim his dad was scumbag Frank Marshall or the racist Malcolm X but only the drugged out dummies or the dimest-witted demos will buy that BS. Would you want to wage a civil war manned only with allies of drugged out dummies and dim-witted demos?