You know, I'm pretty sure it's sometimes used that way, but I'm also pretty sure that there's an actual category of discourse that involves men explaining things in a tone of certitude but from a position of ignorance.

This is part of humanity. It's not unique to men.

I've experienced first-hand the equivalent phenomena where straight people try to comment on what they think my situation is like as if they know what's going on, and end up being completely wrong.

Being bisexual, I know exactly what you're referring to. However, again, the typical mind fallacy is not unique to straight people, or men.

Now I'd agree that the term has become inflated sometimes to mean any negative male reaction to a female narrative, but that's just an argument for deflating it, not an argument that the inflation is universal, and that legitimate examples of illegitimate negative male reactions don't exist.

The issue with this argument is that "male" doesn't belong in your last sentence. Illegitimate arguments exist. Point out why they're illegitimate. If you can't, you have no business responding to the argument.

"Mansplaining" is sexist. It's kind of like the term "hysterical." Cognitive dissonance is encountering somebody who regularly uses the term "mansplaining" complaining about the sexist origins of the word "hysterical."