About sexism part: Imagine what would happen when the same thing happened to a male panelist from female moderator side?
Nothing. Nobody would care about that. Except some panels overrepresented by women and about any SJ topic where a guy trying to shut female mod up to get a voice for male speaker would be literally destroyed.
Guess what, things like that are happening everyday for both sides but for one direction there's recently some overheated sensitivity. I think there's problem where to draw the line, so we don't call everything sexism or look for the context of gender.
I think it would be sexist to not invite good female scientist simply because she's a woman (and invite worse but male guest).
Now about the real issue which is asexual - ability to be great orator / speaker.
Most of great speakers would made their way into persuading moderator to let them express themselves (like you can't simply get between words of Lisa Abramson, Isabel Aguilera, Lawrence Krauss, Richard Dawkins or Neil deGrasse Tyson - they simply won't let you). There's much more speakers (both male and female) who simply are weak debaters and they find themselves much more comfortable in quiet lab than in heated discussion. Regardless of merit they posses.