Johnson would have promised anything - anything! - to be PM.
I don't believe he actually wanted Brexit, as even he is not that stupid. He wanted to lose, and then get elected as Tory leader by the swivelled-eyed members, who would have seen him as one of their own. His face, on the day the result was announced, said it all. He looked like we would all do if City lost the PL in the last match by being beaten 5-0 by Scousepool!
But he still wanted to be PM. And the way forward was to be a tool of the nutty-as-squirrelshit ERG, to bring down May, on the basis that her deal wasn't Brexity enough. He didn't care that he was setting up an untenable position that someone would have to sort out. (And still will.) He just wanted to be PM. He's got the pension, he will have the book deals and lecture tours, he's still admired by the sort of people who think Nigel Farage is a god. He's laughing his cock off.