No idea if the POSIX subsystem used NTFS or some other filesystem but if it was NTFS it probably just used the same reparse data buffer. It's just that Windows only added a symlink buffer structure in Vista/2008. You can manually use the same data buffer in older Windows versions it just won't know what to do with them just like all the other reparse data structures.
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.
