The irony is that the stereotype only persisted so long because no one in the "main stream" ever investigated sooner. You could probably blame the rise of comic movies and Dr.Who for making it popular and therefore giving people a reason to look and see that it isn't all guys (which it hasn't been for a bit longer than this author is likely aware).
Assuming I misused the word "irony" a second ago, the real irony is that attacking gamergate like that it playing to the stereotype they just tried to disprove.

"it's hard to believe it has any defenders"

Maybe Gamergate has supporters because some people can see that "nerd culture" is a fine enough place as is and doesn't need to change itself to appeal to SWJs who will never pick up a video game or comic in their life.