Here is a link to some official lower level documentation. I don't know if it is WRL though, but it really seems to.
From some other persons investigations, it really seems that WinRT is currently built over Win32. Combase.dll and others rely on user32.dll and other classical libraries (MinWin is mentioned). The window that contains metro application is a classical win32 window. DirectX is probably used to draw controls in it.
It is funny that technically, WinRT really looks like FireMonkey from embarcadero: an UI library based on DirectX/Win32.