Think about where the term literally stems from. A smoking gun may tell who did something and what he did. I have trouble imagining a gun (smoking or not) explain why he did it.
fact that serves as conclusive evidence of a crime or similar act...
The fact that I'm asserting is that men do not suffer these kinds of attacks suggest the individual was motivated by sexism. I can't say anymore other than to say that you are wrong. I don't think it means what you clearly think it means. Sorry, no snark intended.
I'm suggesting that these sort of attacks on men are extremely limited is enough evidence to suggest the perpetrator is likely to be sexist and/or a mysoginist, which is pretty much the definition of a smoking gun as illustrated.