(As other commenters have pointed out, Unix paths do not require a specific encoding, so robust applications cannot rely on any assumptions about encoding of existing files. But when creating new files, they must not try to encode paths as UTF-16.)
That's the point. The separator is not "/", because "/" would be a character to encode. The separator is a specific byte, and so is the terminator.
> Valid Unicode paths are encoded as UTF-8 on unix systems.
There is no such thing as "unicode paths" on UNIX systems, valid or invalid.