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You’re right, they’re not. decentralized doesn’t imply federation, but federated implies decentralization.

https://en.m.wikipedia.org/wiki/Distributed_social_network

A distributed social network or federated social network is an Internet social networking service that is decentralized and distributed across distinct service providers (similar to email, but for social networks), such as the Fediverse or the IndieWeb.

Both kind of networks are decentralized. However, distribution goes further than federation. A federated network has multiple centers, whereas a distributed network has no center at all.