I believe you do have to be the author to add "Show HN". Show HN indicates that the maker is present in the thread. If we lose this distinction, Show HN becomes quite meaningless as a prefix.
I was wondering if something like this existed the other day, and I was excited to see this / share it, but I didn't want to put people under the false impression that I'd written this library.
Not sure if you have to be the author of the thing, to use the "Show HN: " title tag... but it would be helpful to people browsing...