The thing is that Windows is actually UCS-2, a consequence of the OS predating UTF-16 by a few years, and coming into existence when Unicode originally thought 16 bits was going to be enough.
For backwards compatibility with old file systems, there's no way the OS can start enforcing surrogate codepoints as forbidden from names. You can just so happen to pretend it's UTF-16 until it's not.
For backwards compatibility with old file systems, there's no way the OS can start enforcing surrogate codepoints as forbidden from names. You can just so happen to pretend it's UTF-16 until it's not.