you have multiple centers, like fediverse/mastodon, where you can choose which center you use, but your account is on one selected server, when your choosed server censor you, or when other choose to censor your whole server you are with same situation as on X.

Decentralised is when your account is not on one specific server. For example on Nostr your “account” is on every relay(server) you choose, so it can by on 100 relays, where you can easly run your own and you are not affected to much if you are baned by one or twenty of them.

Why polycentric make problem smaller, there is still ability to censor you, but decentralised approach make it nearly impossible.