Those simple memory tasks may very well be representative for whetever definition of memory you want to use - as long as that definition is close enough to the definition as found in dictionaries etc, and which is the general and established term for what is meant here.

You make it sound like the gist of this article is obvious, in reality it's not, at least not for everyone. And even if it were you need research to actually prove it, that is how science works.

Calling being able to switch tasks fast, and a lot, an otimization is in a lot of cases a bridge too far probably. It's only optimal if you actually need it. While there are enough jobs which rather benefit from detailed attentaion to just one thing, not multiple. And not all people knowingly train themselves to be able to multitask, rather they fall into it because of addictions etc.