Is keybase decentralized?

Hi,
Glad to see Keybase, but the sparse documentation doens't let me assure if it is a decentralize application or relies on centralized server owned by a single party. Is keybase decentralized? How it compared with matrix.org and Riot.im?

Thanks

The issue asks whether Keybase is decentralized and compares it with Matrix and Riot.im, but names no files, tests, or entry point. Clarify the intended documentation scope and authoritative sources before starting; done would be a documented, accurate answer to those questions.

Written by the indexing model from the issue text.