As far as I'm aware, you've never been able to export a Silverlight resource dictionary, and WinRT is built upon the Silverlight XAML model rather than WPF's XAML model - so you have to export to some Silverlight compatible option.
(As an aside, WinRT on the managed XAML side is basically the next version of Silverlight - though they seem to branched off from Silverlight 4 rather than building upon Silverlight 5)