I think a better term for what you are describing would be a pro gamer. The one that is very good at games.IMO a hardcore gamer is the dude that comes to your house and plays one of your sealed games that's sitting on your shelf for years. Neither person have played the game before. And the person who owns the game gets their ass kicked by someone who's never even heard of the game. That guy that beats your high score 10/10 times and you can never beat his score or beat him in a particular game. That is the hardcore gamer to me.
When I think "hardcore" I think Billy Mitchell and hot sauce. Not some guy with a good income that can afford to buy a lot of games they are never going to play.
In my opinion hardcore gamer does not have much to do with skills. Hardcore has to do with how you approach gaming as a whole. When you get really into it, you know very very specific informations about the industry as a whole or the subgenres you are interested into. Hardcore gaming is about playing a shit load of games and making gaming a big part of your life by devoting a part of your free time to it. It can also be about research, finding the little gems that may have fell into the cracks and playing those obscure good titles that never had their time in the sunshine.
In the end it's all semantics, but it's still a fun discussion.