Please, cite some sources?
Windows runs and has run on more different architectures than any system other than *nix. x86, x64, IA64, PPC, MIPS, Alpha, SPARC, ARM...
Conditional coding means that supporting ARM doesn't affect the x64 code paths, and vice versa. Performance-critical parts of the system can be completely optimized for each target. (Opaque data types help with this optimization.)
Note that Microsoft doesn't add bloat by building fat binaries - each binary contains only the code and data needed by the target architecture. Zero run-time and disk overhead for multi-platform support.
Your statement simply does not recognize the current state-of-the-art in multi-platform programming.
Also multiplatform programming tends to generate robust software. Inserting architecture (low level) hooks can easily ruin software portability. Also, this can lead to more robust drivers because peripheral manufacturers can't presume x86 architecture anymore on device hosts.