Under "Early Childhood" it states: "His birth was registered with both the U.S. authorities and the city's British Consulate, with the child thus being awarded both American and British citizenship." In fact registration of a birth has nothing to do with attribution of U.S. nationality, or for that matter, British nationality (given the facts of parentage and the birth of a parent in Britain). A certification of birth is an entitlement (under the UN Convention on the Rights of the Child) and under New York and Federal law. But many births in the past were never registered, and some, typically signed by midwives in border states, have been questioned (typically by the State Department upon application for a passport or (prior to current passport rules) on re-entry from Mexico) as fraudulent. 

 It is questioned (above in this Talk section) whether Johnson actually renounced his US nationality when he said he had, or would do. And it is also stated that renunciation is contingent upon payment of tax. Johnson did not (it is not quickly done, there is a long waiting list with the Consular Sections in London, Edinburgh and Belfast) but I suppose he will do. The tax issue is more complex: there is an exit tax, quite onerous for "covered expatriates"  generally speaking those with assets over $2 million or an tax bill of $155,000 (as of 2013). There are also other tax implications relating to gifts and estates. There is a concession for persons born to alien parents who renounce by age 18-1.2: https://www.law.cornell.edu/uscode/text/26/877  

 It is quite possible to be a non-citizen  even to have been deported as an illegal alien  and still be liable for U.S. tax, perhaps indefinitely, Indeed long-term green-card holders are also subject to the exit tax system.