It's decentralised as in federation. There is no central host, only instances that users can pick from (or self host). And to make it more than just a self host

It's decentralised as in federation. There is no central host, only instances that users can pick from (or self host). And to make it more than just a self hosted service, those instance hosts can link their networks together and allow users to interact across the greater web of federated instances. So it's decentralised in the sense that Mastodon or Matrix are.