I’m not sure that such is the full on requirement. All of the explanations which are not “Man tells woman how women work” seem rather over-the-top. It’s not impossible that any of them might completely ignore the credentials of a man, but it seems unlikely to me that the expert being a woman didn’t factor in. Even we Dopers tend to listen when someone points out their expertise—unless we have reason to doubt it.
Plus there’s just the fact that the women involved have more context from the interaction and more experience dealing with the topic. We’re just getting this tiny tidbit of information, often not even the words the guy actually said.
Finally, I could argue that it’s not really the woman’s job to figure out if the guy treats everyone else that way. If it walks like a mansplainer and quacks like a mansplainer, it’s reasonable to assume it is a mansplainer.