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
I’m not sure it means the apps will be interoperable, only that the protocol for sharing keys will be standardized.
I highly doubt that we will see any kind of standardization in the actual messaging protocols in the next 10 years.