You tell me. Why did he use it in the past?
My guess. His parents lived in America and he travelled with them on an American passport. The word "cool" in his essay seems to indicate he did this as a teenager, but I may be reading that in. Then he/his parents moved to Britain and he naturalised. Not terribly sinister, if so.
There are almost certainly excellent biographical notes about Mr Johnson on the Internet - for anyone who wants to explore this storm-in-a-teacup further