Well, its all in definition. If they defined "multitasking" on the basis of people being continuously preoccupied with tv, mobile phones and computers then the definition of multitasking is limited to those actions and that meaning. "What is multitasking" is a question that deserves a research on its own, if not a whole philosophical debate. But it doesn't work that way in these kind of researches. The definitions are often very poor, not very broad-sentenced (that would take alot more time and resources to actually research etc) and don't often live up to the complexity of it's common meaning. I see it all the time, valid scientific statements based on very poor definitions. "People who think they are good at multitasking are actually really bad at it" has some sort of scientific truth, but common sense goes "ORLY?!" It says so very little, because its definition is so small. The impact however can be enormous, that's the worst part. Scientific research can change peoples belief, and with the change of beliefs it can change peoples actions, their attitude. Most of the people do not have the brains nor knowledge or attitude to critically look at any research, so i wouldn't be surprised if some people would actually think that multitasking=bad. Or something like that.
Anyhows,
On the subject of attention. I did a module of NLP in school and at some point i read that people can only process about 9 blocks (or less, cant remember exactly) of information. Meaning that your attention/focus cannot be on more than 9 things at the time, and the more blocks you process, the worse your attention for the individual blocks becomes. For example, i'm writing, while actively thinking about what i want to say, translating from dutch to english, checking grammar and sentence structure. It's taking me a while to write this post since i'm also paying attention to the cats on my desk for example. If i didn't need to that, words would flow out of my fingers more fluently. And they do ever more so when the cats leave, or once i feel i start getting better at translating, for example.
Ofcourse theres also alot of stuff people do unconsciously, they require no conscious attention. Take walking for example or moving in general, breathing, letting your heart beat, having thoughts spawn into existence. (Focussing on thoughts is something different though)