Even with Nokia adopting Windows Phone, I find that very hard to believe.
Microsoft is offering their OS to all the hardware vendors "out there" and with Nokia, the largest hardware vendor becoming an exclusive Windows Phone partner I don't see these numbers as being unrealistic.
Of course, 5-year out predictions are hard to do, but I think it's realistic for mobile operating systems that are available to all hardware vendors will be more numerous than the OS platforms that are proprietary to a single line of devices (iOS, Blackberry, webOS).
I'm not saying they're right, but when I read their numbers I think "yeah, looking through the visor of 2011 this could certainly happen".
In order for WP7 to have a significantly smaller market share than the IDC prediction, Nokia will need to totally flop when it comes to selling Windows Phone handsets.