It's well hidden, but the relay system is the centralised part.
The posts you see come from the AppView. The AppView gets its data from the Relay. The Relay gets its stuff rom the PDSes. So you always go through the relay. No direct connection from AppView to PDS. Yes, they promise that in future you might be able to run your won relay. But right now and since its inception, Bluesky rund the only relay system you can connect to. I call that centralised.