What does post/comment permanency have to do with privacy? The linked post is an opinion, with no facts backing up their extreme claims.
It's true that if you delete a comment, your username remains, but is that a matter of privacy? Was it acceptably private before deletion? Why does that change afterwards?
I'm extremely skeptical of the poster sharing partial truths with opinion and no sources.
Edit: I read some of the comments. Poor jorgesumle4, yeesh!
Edit2: and won't be going anywhere near raddle, either. Oof! "My echo chamber isn't echoing right! I must now yell and spew some ad hominem hate! Ahh, much better."
Edit3: To @orientalsniper@lemmy.world, I would say your OP question, "is there truth to this?" is being asked about an opinion. Can opinion be true or false? I don't really understand the premise. If you're posting on the public internet, that's not private. Full stop. Any platform on the public internet, no matter how you can or cannot delete your contributions, is not private.