It is not even semi-private. It is a completely public medium and absolutely nothing posted on it, including direct messages, can be seen as even remotely secure. Worse, anything you post on Mastodon is, once sent, for all intents and purposes completely irrevocable.
This guy is either actively trying to spread fear and doubt about decentralized services, or is somehow only now understanding what the internet is and how it works. Did I step into some kind of time vortex a while back and end up in a world where people ever believed that anything on the internet was private or revocable?