Might this (mansplaining) be related to this (men on average being more narcissistic than women)?
I would speculate yes.
By far, most men who have engaged in mansplaining with me seem to have been motivated by supreme over-confidence in their own intellect coupled with an assumption that my own, a woman's, was beneath theirs. They have also seen driven by entitlement, both to my time and to engage in aggression when they have deemed me insufficiently pleasant to their attempts to "correct" and "instruct" me.
This being said, I would agree with the speculation that men being, on average, more narcissistic than women is at least partly explained by socialization.
Not all men. Of course.
I also believe that men's on average *ahem* greater confidence in themselves also contributes to women experiencing imposter syndrome.
In my professional life, I have learned to be wary of those, especially men, who often speak with extreme confidence and who rarely express doubt, even when complicated issues arise. I have also learned that, when people are uncertain in professional situations, people who speak confidently can be very convincing, even if they're completely full of shit. I have seen leaders make horrible decisions, because they listened to the persons who spoke the most confidently rather than the person who expressed uncertainty about a course of action.
When I have been faced with decisions in my own life, I try to remember this, and to therefore trust my gut and listen to my relatively shakier inner monologue that's thinking we maybe don't know all of the facts - even if there's a louder, more confident voice saying otherwise.