I haven't experienced a person who misuses "mansplaining" in the way you're describing. I'm sure it happens, and I don't agree with those who do that, but I have witnessed the situation the word is meant to describe several times too. I'm more inclined to think that it still happens all the time and is accurately pointed out by the word.

I agree that someone willing to condescend to women will be more likely to condescend to a junior employee or a teenager (i.e. they've already shown the propensity for prejudice), but I don't agree that they are probably condescending to everyone (I'd argue that kind of person is very rare).

It's not that there are legions of very sexist and evil people who are not prejudiced in any other way, it's that there's an unfortunately large spectrum of mostly well-meaning people who might harbor conscious or unconscious beliefs about women specifically. They might also harbor similar beliefs about minorities, or have a life experience that justifies their beliefs in their eyes, but nevertheless it's a big enough cross-section of society and a strong enough phenomenon that it's been noticed. That there is some subset of SJWs who think that they can take advantage of this isn't an extreme claim, but that it's a stronger phenomenon is a pretty extreme claim to me.