This is a subtle point, but that thinking is misguided. You, as a client, have no control over what server you’re actually talking to. The only way to be sure you are secure is to be secure independently of if the server has been hacked, or is malicious, or whatever. Thus, you design your system such that the client only discloses information that is allowed to be public to the server (public keys, encrypted messages where the server can’t decrypt it, etc). In that way, you don’t care what code the server is running, and auditing the server makes no difference to security.

The whole purpose of cryptography is to reduce the set of things you need to trust. Including the server in the trusted base is not only a worse design but a false sense of security if your trust ends up misplaced (hacked).

So, this is different than how a bank would work, for example. In the case of a bank, you have to trust that their servers are secure, and their software being open would help with auditing that. In this case, keybase is not the endpoint you’re talking to: another keybase client is. The keybase server is just an intermediary, just like any router on the internet.