I'm suprised no one has brought up that men talking over women is a well-studied phenomenon in childhood and adolescence. Girls tend to contribute less to discussions or conversations in mixed gender settings than they do in single-gender settings. My parents even noticed it when my brother and I moved out of the house, and suddenly my little sister was a lot more talkative at the dinner table. The exact mechanism or reason for this is not well understood.

I'm not sure if this phenomenon has been studied in adulthood, but I wouldn't be surprised if some remnant of it remains. Could this simply be an example of that phenomenon in action? Which isn't to say this isn't sexism, I wouldn't consider those two things mutually exclusive.

EDIT: to be clear, I'm not absolving the moderator of blame, or placing any fault on the woman. I'm simply linking this behavior to a broader social phenomenon that is fairly well studied. I personally think what he did was sexist, whether or not that was his intent.