The idea here isn't that you use keybase to find out Maria's twitter, github or gmail identities - it's the opposite. The idea is that you already know who Maria is on one or more of those services, so the fact that the account you know is Maria's at github has posted a signed message from that public key is supposed to testify to you that that is really your Maria's public key.

You could of course manually review and verify Maria's github post that contains her public key - all that keybase is really doing here is providing an easy way of discovering that github post (or tweet, or whatever).