Long time geek, fangirl, mother, and reader. I've got a lot to say, you might not like it all, but it will be honest and hopefully helpful.
"Twifans were not coming from subcultures; they were often not self-identified geeks; and importantly, they were also more likely to be older, professional women with professional-grade skills and resources."
It's important to also note these older, professional (aka middle to upper middle class), white women were in the majority in the part of the Twilight fan fic fandom which focused on erotic romance.
These women do not represent ALL of the Twilight fan fiction fandom. Their corner of the fandom was the most active and popular because they produced a steady stream of (FREE) erotica/romance.
Which is significant when you look at the publishing demographic that is supporting the sales booms both in YA and Erotica/Romance. These women took the model of fandom culture and fan fiction, wrote the stories they wanted to read and sold it to each other.