I was trying briefly to think of subgenres of SF which really are completely male, and I couldn't; even space opera has Lois McMaster Bujold (although it's very *odd* space-opera!), and alternate history has Susanna Clarke.

It might be difficult to write convincingly for the other gender, but enough people do it (both ways round) that it doesn't seem to be a problem. The most generous reading I can put on his argument is that male/female writers writing in a female/male-dominated genre might write "unusual" works for that genre, but it's not even clear that that's a bad thing!