Unix filenames don't pretend to be something they aren't. Windows filenames like to present a convincing façade of being UTF-16 right up until they aren't.
Except they never have? Unix paths have always been bags of bytes, both before Unicode and UTF-8 were invented and after. It's just convention that modern Unix systems use UTF-8 as the text encoding for paths.
And neither have Windows paths ever actually pretended to be UTF-16, that's my point.