It will be open source, end to end encrypted using Signal’s double ratchet encryption protocol, and he plans to make it easy for fediverse platforms to integrate it. The beta will release later this month.
He’s also the creator of https://fedidb.org btw
“Private” is not really a binary concept.
They’re “private” in the sense that no one can see them other than the participants and the server admin (if they really wanted to).
They’re not private in the sense that they can be hacked and leaked, or subpoenaed.