Just going back in history when we had LAN games.
Remember Diablo2? There we had a HARDCORE mode, where you lose your character when you die ingame...
I do not say its the earliest who used "hardcore gamer" but it helped to form the definition back then!

The "need time to play" in the definition of "hardcore gamer" is based on that fact of losing all when dieing ingame. You had to start anew and all time spend with previously character is lost...
To argue with "hardcore = time you spend ingame" is prove of not knowing history, same as when saying "hardcore = difficult"