there are men who assume women are less informed and less knowledgable about something because they are women

Yep, and there are women who are too overbearing but I don't go around telling every woman who speaks with a bit of authority that 'she should tone it down because women are too bossy'.

Because that would be shitty.

There are women who too often rely on emotion rather than reason, but I don't tell every girl who wants to tell me how they're feeling that they're 'just being irrational- which is just like women'.

Because that would be shitty.

BUT SOMEHOW it's all well and cool, when a guy engages in a discussion and shares his opinion or experiences, to label him as a 'mansplainer'.

'b-but what if the guy is actually being an asshole? then he totally deserves it!!1'

In a situation where a female friend gets a ticket, it's a huge dick move to say something along the lines of 'it figures, that's so typical of a woman'. i.e. even if the charges are valid and statistics support the observation, YOU are the person being adversarial and reducing the discussion to a snide ad-hom against an (irrelevant!) group the person is a part of. It's a shitty thing to do, and it's baffling how so many people that are ordinarily pro-gender-equality seem to love to use the term.

TL:DR You're just being a dick and I think, on some level, you know it. Why is it so hard to say 'actually, I already know about this'?