I certainly know what people are talking about, and though no one has accused me of it in my hearing, I imagine the word has been breathed behind my back. I am overdominant in conversation, taking almost every other comment no matter how many are present. (I have compensating virtues of listening, reflecting back, affirming, and remembering that surprise people, but I doubt those fully balance the scales.) My sons and my brother can give me a run. Someone needs to say the most important thing next, and in the moment, that is often me. There are rare occasions where I will sit quite comfortably listening to some other person speak, even at length, and even if they aren't very entertaining (though I do tend to nod, murmur, and insert single words or phrases that I think are clarifying if they don't express well). I want the best comment to go next. They clearly have it and I don't. The true examples of mansplaining, of explaining something to a woman when she knows the subject better than I do, happens, but not often. It doesn't happen often when I am talking with men, either. It's not so much mansplaining as Wymansplaining.
I can think of only two examples of women using the word mansplaining at my place of employment, both times with a polite apologetic nod to me as they said it about another man. In both cases, they did not know more than the offending male. In one instance she knew far less, in another, he simply had a different opinion which she didn't like. That is not a sample size that counts for anything, so I wouldn't put much stock in those results as an estimate. However, it is evidence that both those purported alternative explanations do sometimes occur. I have observed resentments and hurt feelings from episodes where the word was not used but some comment about maleness being part of the problem did come out when the person left the room.
For sheer numbers, the interesting bit of information where I work is that 90% of the employees have a female direct supervisor, and the supervisors above that are also 90% female, all the way to the CEO. The medical director, who actually works for Dartmouth-Hitchcock, is male, with half the medical staff of two dozen female. Nursing, social work, psychology, rehab, legal, medical records, and housekeeping all have nothing but female supervisors, all the way up, all the way down. The complaints that I hear with specific reference to maleness are all at upper levels of power, among more aspirational, ambitious women - who are often also pretty smart and competent. At lower levels of power people simply say that someone is a prick or a bitch, both of which have "gendered meanings," but I think are rough equivalents.
I don't have a theory or conclusion about this, just noting what I observe in my fairly unusual setting. I think my experience undermines the current narrative, but not in any particular direction.