Decentralization is a loosely connected network of peers not reliant on one or the other. The loss or gain of a peer or peers is of no significant consequence to the peers at large.
Centralization is a network of peers all connected to one hub, reliant on the hub to provide spokes to other peers at large.
The "fediverse" as far as I can tell is a collection of many separate networks that refuse to speak with other networks (for ideological reasons at that, rather than technical), each network acting as a hub and providing spokes to their peers.
It's centralization ("Lemmy", et al.) within a centralization ("fediverse") within a decentralization (internet).
