Technically, it is NOT true
When you compile a .NET application for the "Any CPU" (the default), you aren't targeting x86, x64, or ARM, you are targeting the .NET CLR. The JIT compilation only is for ASP.NET, standard .NET applications do NOT JIT. So no, you are *not* recompiling a Metro/WinRT application for different architectures, either at run time or at build time.
J.Ja