What distinction do you mean? Federation allows for interoperable decentralization. Without federation, we would have thousands of chat/mail/social media servers that can't talk to each other. Some may choose not to federate, but most want to federate to create a useful protocol.
I always believed decentralized was if each user doubled as a server and required no external setup (example: scuttlebutt). Whereas federated was a plurality of servers with users communicated with each other but no central authority (email, mastodon, matrix, etc). However, reading some peer to peer literature like that 1500 page behemoth of a book, "Handbook of Peer-to-Peer Networking", it seems they are used relatively interchangeably..