I'm still not entirely sure what problem Sup is trying to solve. Matrix already exists. Matrix supports E2EE through the signal protocol, as well as native federation, and it bridges to almost any existing chat service. Matrix is inherently less secure, overall, than Signal, but I don't see how Sup would fix this either -- for that I'll have to wait and see. As for using one's fedi account to sign-in, that's mostly just up to supporting OAuth, and not some feature that would be unique to the app.