The author is correct, Metro is the heaven for all those grandmas and family relatives who before would call/mail us on "how do I power on the computer"-type problems. Now, you put 'em in front of a Metro PC and boom, your incoming calls reduce by half.
Sure, she doesn't know how to "use" it in the grand sense that we savvy users might. But, she knows how to do things. She knows how to Facebook, she knows how to send an email. Unlike us, she doesn't think of these things as general concepts. They're very specific, down to the smallest detail. Email for her is hotmail.com
Now, grandma is presented with Metro (or as I like to call it, The Boxes). The small, basic knowledge she gathered over time that gave her freedom of communication is now worth nothing. Perhaps she'll explore, and perhaps over time she might find that the interface is in fact easier to use. The problem is she wants to send an email now, and none (and I really mean NONE) of the anchors, symbols or paradigms she's used to even exist anymore.
On a tablet, this makes sense. She can intuitively poke around and maybe get to the email app. She's not used to touching a screen, and the opportunity gives her a chance to learn something new.
With a mouse and keyboard, she doesn't know what to click on. She's never clicked on text and had it perform an action. "Charms" bar? Corner of the screen? Ha.
Now imagine we're not only talking about grandma, but instead average non-technical office worker. Not only does said office worker need to send the email now, she needs to send a dozen yesterday. She has no time to learn something new. Her company paid good money for a device that's supposed to save time.
I agree that this article gets to the bottom of the issue, though. Microsoft has underestimated its users.
A lot of win8 is not easy to figure out by yourself if you've only used win 7 before.