Hm, so it's encrypted from your beeper client to the bridge, decrypted, then re-encrypted with the outgoing platform's protocol. Seems like a good reason to host your own bridge, and a good call on it being a glaring attack surface.
Seems like the secret sauce is in how they deal with messaging platform integrations? Maybe the goal is to avoid another iMessage lawsuit. With Beeper as a proof of concept it would be cool to start adding integrations in a fully open source way (legality permitting)