There is no guarantee of anonymity for anything you do on the Internet. Same with email.
The constitution guarantees your right to free speech, not the right to be able to post things then be totally immune from the fallout from those postings.
If you were standing on the street corner yelling obscenities to everyone who passes you can certainly do that, however you must be willing to bear the response to your speech. The people who posted (and I have not read the article) probably thought they could do so and not ever hear any more about it. They were wrong plus they didn’t read the terms of service.
Everything you do on the Internet is logged somewhere.
When the “Thought Police” start searching for “unlawful thoughts” on FR in order to find “counterrevolutionists” for re-education camps or elimination, will we be protected by our pseudonyms? Naw, never happen here!