Goslin thinks "the big difference between casual and hardcore gamers is the amount of time they are willing to invest. To attract the former, you have to get them engaged faster, because their time is limited. Once they're playing, however, the game needs to be challenging, deep and fun, if you want them to continue. If you succeed in creating a game that's challenging, deep and fun for a casual player, it will likely also be fun for a hardcore gamer."
While this is in part a simple question of terminology, this statement is still fundamentally incorrect. The primary difference between hardcore and casual gamers is their willingness (and desire) to fully explore all the options a game provides. It is also a question of skill and experience.
Time is not a factor, in my mind, when determining the difference between a hardcore and casual gamer; hardcore gamers may be more willing to dedicate a lot of time to a game in general, but casual gamers can spend a ton of time playing games, and hardcore gamers can spend little.
For example, I am a hardcore min/max style gamer. I almost never spend more than 10 hours a week playing any given MMO these days, but the time I do spend is carefully approached and maximized for efficiency.
On the flip side, while I was a Community Relations Manager on EverQuest II, I met a number of gamers I would consider casual, in that they spend most of their time in the game hanging out and slowly advancing, but they played the game 4+ hours a day. They never maximized efficiency, watching TV while playing or wandering in and out of the room in which they played, casually chatting with friends, looking around, etc.
I understand part of this can be that what I view as a hardcore player has different goals than a casual player (character advancement being one of them), but I look at a casual player as being someone who simply doesn't put forth a great deal of effort while they are playing a game as compared to a hardcore player (whether their motives are character advancement, monetary gain, socialization, or otherwise).