So rather than use the most popular platform (windows) to create a "mock-up" of this device they chose Max OS X? I don't think so.
the text says they want to be prepared for anything apple (or anyone else) has to offer.. so why not use mac osx theme? but that does not proof that apple is working on a tablet, nor that it's going to be released anytime soon.. (I think they are working on a tablet^^ but that doesn't matter
It DOES actually run, it is a running demo, if you read the original source, or the AI report. Just with a flying hand over it, which makes it look like a mockup.
the author said he was "able to play with it"... he didn't say anything about the "internals" of whatever he had in his hands.. we don't even know if he got anything in his hands at all. It could very well be a "demo-app" which is controlled with the mouse, it doesn't even have to have a touchscreen.. like the iphone emulator in Xcode or the android emulator for eclipse (...)
There are plenty of possibilities to create prototypes of "programs" you are going to create... you click on a certain point and it opens a predefined activity which then again shows texts etc... like an animated slideshow where you can chose which "part" you'll see next... call it an interactiv slideshow with various routes to go...