A good example of the same thing would be if a woman that complains a lot and just seems to be in a bad mood is told she's "PMSing." Statistically it may be accurate, but the assumption is rude and sexist.
It's asking us to think about why the term bugs us, as well as how helpless it feels to be unable to stop being dismissed with terms like these no matter how much you explain it. We all have to change.
"Mansplaining" doesn't hurt my feelings or oppress me and my expression at all. This isn't necessarily true for the hundreds of other words we use to dismiss women and their behaviour.
"Mansplaining" doesn't oppress you like "PMSing" doesn't oppress women, which it does. It reinforces negative stereotypes and forms potentially false assumptions. Just because other people say mean things about women doesn't mean you should say mean things about men.
EDIT: this could be seen as a rhetorical jab, but I really don't mean it that way, I'm just not sure in what context that would be insulting.