That depends on what “centralized” means (i.e. whether we're conflating “decentralized” with “federated”, as is usually the case when talking about systems like ATproto, ActivityPub, Nostr, etc.).

In any case, AFAICT it's possible to self-host every part of the ATproto stack, and people have indeed done that. Third-party PDSes, relays, and applications/clients can talk to one another and with the ones Bluesky PBC operates. That satisfies the meaning of “decentralized” as most people use it (at least as well as other federated protocols like ActivityPub and email satisfy it).