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
To be a bit more precise, Signal is against federation from two angles:
Innovation: Signal values absolute control over the protocol so that they can more rapidly implement UX experiences scene in other modern messaging apps. It also eliminates malicious or outdated servers changing the UX between users. Ultimately folks won’t blame the servers, they’ll blame the app, and stop using it.
No rope for users: They seem pretty confident that the Apple-style of software and UX is right— if a user can change stuff enough to break it, they will. For secure messaging, they’d rather users have fewer choices to be sure it is secure.