That maxim is no longer true, especially in the medium-to-long term. Reddit posts older than 8 or so years old aren’t accessible (even to their creators), same with the original Digg posts. If you go back to the Usenet days, most posts haven’t been archived and are lost, especially those from the smaller newsgroups. I expect over the coming decades, we’ll see data loss as a growing issue.
Immediate flaws, I can see:
Cameras (or human observer) undo any sense of anonymity. A bad actor could link participant with account.
What’s preventing a MitM attack, where the BBEL (Big Bad Evil Librarian) substitutes the participant addresses with bot addresses?