1. I wouldn't think of Ann Leckie as a "girl". She is unquestionably an adult woman.
2. Military SF written by women is not even that unusual. Over the years, I've read several works in that genre that were written by women, including some long series. And I'm not even that into military SF! Had that panelist been ignoring them all as "girl stuff" until one won a Nebula? It boggles the mind.
3. Even if this WAS unusual, pointing out that it was written by a woman is a silly way to promote it, opposed to saying that it won a Nebula and is really good. Unless you are trying to point out that it's military science fiction that isn't horribly sexist like a lot of the stuff men write, which would be a valid thing to note, but not really what the guy was saying.
I haven't read "Ancillary Justice yet, but I expect I should, it's supposed to be really good. I don't know much about it yet though, since I was so dumbfounded I tuned out the rest of the guys recommendation.