It's BS, that's what it is. I still think of hardcore gamers as the "educated" and people who don't ONLY play the big pop culture games (Smash, Zelda, Halo, Madden, CoD, you know). A hardcore gamer just focuses on what are "good", outstanding and/or original efforts in videogames, whether it's tracking down Panzer Dragoon Saga/Radiant Silvergun on eBay or getting into GTAIV.
Casual gamers are people who don't take the hobby seriously enough to really know what games are out there. There's nothing wrong with that really. But people seem to think that buying a lot of games, often more violent ones, qualifies as "hardcore". And graphics can be as important to you as gameplay, but a hardcore gamer knows not to outright dismiss any game based only on graphics.
I have no idea how these terms started to be used as such. I think it was a reactionary thing, against the Wii, honestly.
But just doing something a bunch doesn't mean you're "hardcore". Playing Street Fighter II everyday doesn't mean you're a "hardcore" player if you never get better or explore the gameplay. Hardcore is supposed to be an avid fan and in-depth knowledge of something.
