HWNDs are actually a bit special. HANDLEs you get from CreateFile and so on are implemented by the NT kernel and are real capabilities (like Unix fds), so the actual bits of a HANDLE from one process are useless in any other one.

On the other hand, HWNDs are global (at least within a window station, I don’t know how that works). If you display the numeric value of a HWND in a message box in program A and type it by hand into program B, program B can (usually) pass it to PostMessage and successfully reach program A. I don’t know if other USER and GDI “handles” (e.g. HMENU, HRC) are like that, but I wouldn’t be surprised if they are.

I also think defining STRICT before windows.h will still prevent you from passing a HANDLE where an HWND is needed, even though the last (also the first and the only) time Microsoft promoted this feature was in the Win16-to-Win32 migration docs in the early 90s. A HANDLE to a file vs a HANDLE to a named pipe or even a mutex is another matter—those are all genuine NT objects, and handles to them are to an extent all the same kind of thing.