Although they didn't come out and say this, the impression I get is they are bringing over the WinRT APIs for lower-level stuff like networking, etc., while the XAML UI stuff is an extension of the Silverlight in WP7x rather than a port or adaptation of Windows.UI.Xaml from Windows 8.

If that's what they're doing, I think it's the right decision - the UI conventions themselves are different between the phone and Windows 8 (more than people realize, despite having a similar visual aesthetic), so you'd have to rewrite much of the UI anyway - so why break compatibility with WP7.x.