Libel and slander don't have to be public to cause harm or damage. What they have to be is created with the deliberate intent to cause harm or damage.
Not created with the intent to voice contrary opinions, rant, piss, moan, whine, vent, mock, deride, or call names, but specifically and solely with the intent to cause harm or damage.
The legal bar for proving such intent is very high, precisely for the reason that individuals and institutions historically try to use the law to inhibit criticism of themselves. The more powerful the individuals and institutions, and the more antagonistic those who speak against them, the more varied and subtle the attempts to inhibit the speakers -- as the writers of the First Amendment and framers of libel law knew all too well.
The Philly case is a blatant attempt to inhibit critical speech. Had anonymous writers been praising the panty-bunched peeps, there would have been no demand for them to identify themselves, much less legal action.