Due to the fact that Johnson is a renowned liar, I find his claim about Putin threatening to kill him with a missile hard to believe.
No one can back up the claim and I get the impression that Johnson is using it as another attempt to fuel his misguided self belief that he is a brave statesman, who is putting himself on the front line for the sake of the country.
Everything he says is said to try to benefit himself and the public's perception of him.