It's funny, I never considered 'amount of time played' all that important when defining 'hardcore-ness'.
I play WoW. One of my friend's girlfriend plays WoW. She probably plays at least 10 more hours I week than I do. But I'm the guild leader of a raiding guild who has everything on farm and who has been doing hardmodes, going for achievements, etc.
Meanwhile, she's in what I'd call a 'casual' guild, barely even clearing the first Tier of raiding instance (of which they are now 4) and who spends more time collecting pets, mounts and various vanity items. Yes, she plays a lot... but she's essentially acting as as if it was a single player game. Shamu's recent [http://www.escapistmagazine.com/articles/view/columns/experienced-points/6921-Experienced-Points-Massively-Single-Player-Part-1] article really speaks about people like her.
Now while I don't pride myself in being an hardcore gamer (More like I'm addicted to WoW), I definitely don't think she is an hardcore gamer either. Time played plays a factor, but so does what you actually do with the game. You can have a rather elabortate discussion about Warlock class mechanics (the class shes plays 40hours+ a week) and she has no idea what you're talking about... Her talents are terrible, she's gemmed poorly, she has no idea what kind of stats to go for - in short, she's a noobs who plays 40 hours+ a week. I'm not saying all 'hardcore' girl gamers are like her but this research would consider her an 'hardcore gamer'.
Not to mention that it's one thing to do this study on a MMORPG, but what about fps, rts, etc?
Not enough information to draw a conclusion really. I suppose it's kinda fun to say 'girlz can be hardc0re too!' thought.