I also didn't really understand his standard. First, I think that we should just discount difficultly entirely when talking about how 'hardcore' a game is - modern games just aren't that difficult. You really don't need to be particularly intelligent or skilled to beat Zelda, and I've seen people who play very, very casually pick up Gears of War for half hour periods. Modern games simply aren't that difficult; most are incredibly forgiving, and they're packed with hints as to what to do next.
What I find really odd about his standard is that I've tried his 'experiment'. My girlfriend and her roommate (and her roommate's non-gaming boyfriend) devoured Zelda: TP. A female friend of mine back home loved to play it on her brother's Wii. Not a single one of them will play Smash Brothers. At one point or another, several have shown interest in the game, but they quickly realize that becoming good enough to play with those of us who game regularly requires more than they're willing to put in.
I'd argue that the opposite standard would be more accurate. Casual gamers prefer single player games where they don't have to compete against better players. Casual gamers prefer games that don't rush them, where they can take as long as they like or as many tries as they like to accomplish something. Casual gamers prefer an open world with nonlinear objectives to a 'go here, do this' setup typical of many modern FPSs and RPGs. Casual gamers like to play; they don't like to compete. A game will tend to appeal to a casual audience insofar as it advances elements of play and freedom. Multiplayer games are inherently less appealing to casuals because they add elements of competition and constrain free action - you're not the boss of a multiplayer game; you're only one player.
