Had he never used an American passport he would not have had this problem. The question raised is why are you not using that passport now? Has it been revoked or confiscated for cause? No switcheroos allowed.
He uses his British passport. Because he is British. Why would he use an American passport when he is a British citizen? He is not culturally nor emotionally American, nor is he bound in honour to America. He is a British man, and in fact is a member of Parliament. He has a tick against his name because of where he was born and this tick is stopping him from being British - in the eyes of American passport authorities. This is the point of his essay.