Why do you think somebody is a casual gamer if they don't play loads of different games? I certainly wouldn't call somebody that puts thousands of hours into a single game a casual gamer (because they're not). If you're a casual gamer you're spending a limited amount of time on games, that's it. Doesn't matter whether it's 1 game or 1000 games, if you put a lot of time into gaming you're not a casual.
Agreed. Assuming this is what the OP meant.
The time you put into gaming is the clearest indicator of how casual or "devoted" one is as a gamer. Everyone has limited time outside of various duties, and what you choose to do with your spare time reveals what you're really into. Doesn't matter what type of games or how many different games.