Yeah, my current guess for how this will work is that users will do an OAuth sign-in dance with their Fedi accounts to a client that is mostly not connected to their personal accounts in any meaningful way, beyond viewing contacts that have also signed in to the client. The client itself could store the encrypted chat, rather than having it reside on any server.

Eventually, we may be far enough along at a point that the ActivityPub protocol could have an enhancement proposal for supporting chat, and apps like Sup could plug into servers in a more native way, where the conversation data lives encrypted on the server.

This is all just conjecture on my part, but it seems like a viable way to work around current network limitations.