We already know that a Windows Media Center PC can stream HDTV via WiFi to an Xbox 360 via the Windows MediaExtender functionality, so maybe we're sticklers on these kinds of statements.
On top of that, the description of the Apple TV as the "next-generation Mac Mini" strikes us as a little odd, since the Mini was never sold as a mediaextender (even though you can use it that way) and the Apple TV was never sold as a general-purpose computer (even though you can use it that way).
From a software perspective, a Media Center extender does only one thing: deliver the Windows Media Center experience to a TV without a PC attached to it.