The Keybase server manages giving out usernames, and recording the proof URLs for users, and then your client hits the URLs, checks that the proofs are signed with the appropriate key, and caches them to watch for future discrepancies.

Keybase offers decentralized trust, in that the Keybase server can't lie to you about someone's keys -- your Keybase client will trust their public proofs and not the Keybase server -- but it's not a distributed/decentralized service as a whole, because you still receive hints from the server about where proofs live, and learn Keybase usernames from it.

(I work at Keybase.)