I don't really understand how. If the argument by Brexit is that we should forge stronger alliances and connections with Anglophone nations instead of the EU, it makes sense for him to cast some truth and feasibility on the matter. It's not like he got involved with no prompt.
Well, we've certainty been a useful Trojan horse in bringing 'l'anglosaxisme' to the EU, so it's perhaps not surprising that the American Establishment want us in.