Correction: Zulip is not centralized

I think of "decentralized but not federated" as like classic IRC, where there were multiple disconnected networks (so, not federated), but each network also had no single server necessary for having a conversation on that network (users would pick somewhat randomly between servers that weren't too far from them).

None of these options are going to be centralized in the sense of having only one instance, since anyone can get the server software to run their own instance.