The subsystem in question would be the one to handle the logic for the syscall. So the POSIX subsystem would use the reparse data buffer as needed. It's just that the Win32 subsystem added its own symlink one in Vista/2008.

This is all a guess, the POSIX subsystems were a bit before my time and I've never actually used them. I just know how symlinks work on Windows/NTFS and when they were added.