Of course it's already possible, but is it convenient? You're sitting at an airport gate and want to show something to your travel partner on your device. You unhinge the second monitor and hand it to them. That's what I envision based on what they described, not sending them a WebEx link or trying to setup some kind of screen share.
That's not the point. The GP comment clearly referred to using the second screen to share with someone nearby, such that two grown adults don't have to huddle around a single screen, possibly sharing earbuds. If you know where someone lives, why have a phone when you can send them a letter, right?
The point is, if you have two screens on your device, it would be useful to be able to unhinge one (possibly still tethered) to make it more convenient and comfortable for a second person to view what's on your device.
I would absolutely buy something like that for doing in person usability testing with prototypes in settings outside a usability lab.