Can't vouch that this is the case with keybase, but where I've seen this done before, the hash is calculated in combination with some kind of time-based shared

Can't vouch that this is the case with keybase, but where I've seen this done before, the hash is calculated in combination with some kind of time-based shared secret at both at the server and the client and then compared, with the theory being it would protect against the TLS connection is MITM'd; all the attacker would get is the hash, which is then useless as the time element isn't known. IMHO, MITM would be better defended against using HSTS, certificate pinning, and perhaps DNSSEC.