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
Like anything, depends on the threat model. Private from your little sister? Probably. Private from your boss, at least in the next few months prior to them being leaked? Also probably. Private enough?
That’s to some extent a question that can only be answered individually, as everyone’s threat models differ. I suppose this fact (everyone having differing threat models) is one of the reasons that so many arguments occur over security.
In the end any successful chat service is going to be used by horny teenagers sharing nudes with each other, which is honestly for me better reason than any state secret why all communications should be end to end encrypted at all times. I don’t trust Zuckerberg or Musk with that, or any other third party for that matter.