As someone said in a comment on the question, just because hard links to directories aren't permitted (i.e., by the
ln command), does not mean they are not possible. The superuser can actually use the "-d" or "-F" option to the
ln command to force the creation of a hard link to a directory (though the man page says it will "probably" fail due to filesystem restrictions - not sure what that's about, and I'm not going to try it on one of my own systems to see...).
Hard links to directories are not permitted because they can create loops for programs that try to traverse the directory structure. In any directory,
.. are hard links to that directory, and its parent, respectively - these are "well known" special cases and anything that tries to traverse the filesystem knows to account for that. But it is certainly technically possible to create a hard link to a directory if you're persistent - it's just not advisable.