to be cunning takes an intelligence and quick wittedness too. Sociopaths are wired differently, they apply their intelligence in a different way to you or ISemantics maybe, but don't know about "extremely" intelligent. He famously struggles with detail. Admittedly a lot of that is down to his laziness, but the genuinely extremely intelligent people I know, or who have worked with, understand detail without having to try. They get it first time. It's easy for them and it wouldn't matter if they were lazy or not.
Johnson doesn't. He's bright enough, but extremely smart/intelligent? I just don't buy it. He's very cunning though, which is a bit different.