What kind of data is decentralized?

The underlying Matrix Protocol decentralizes on the level of "rooms". Users are bound to their servers (like in email or mastodon) and called a "home server" and data of any room is only shared and usually replicated to the server that have users in that room. Even if you had access to a particular server only all "connections" of users from that server would be know. As such there is no public view or graph or knowledge of all connections between all servers. In that sense both are decentralized in the entire system but centralized with their home server from the perspective of the individual user.