They play hardcore mode of course!
Na but seriously hardcore has many different definitions depending upon who is defining it. From my experience in games across the board (although you hardly hear it on COD), hardcore is simply a term used to describe a skilled player who plays a lot and likes to succeed(mold success into whatever your definition of success is on a video game). For COD success could be winning or it could be a different specific goal of be ranked high on the leaderboards or getting a lot of kills. There are multiple methods for someone to appear to be a hardcore player.
Honestly though, why are people bringing MMORPG terms into a FPS?