XMPP, like email, is not a 'decentralized' protocol, but a federated one. If you and your chat partners use the same server, it would be as centralized as WhatsApp or Signal.
This makes sense, because Baran was describing a communication network, like later ethernet, where nodes were interchangeable, wherever in XMPP the nodes are not interchangeable: if you have an account on xmpp.org server, you can't connect to xmpp.jp server and receive your messages. Thus, such networks required a special more narrow name to define it.
[1] https://networkcultures.org/unlikeus/resources/articles/what...
It is decentralized in theory (no one server controls the network), but 'federated' is the correct word to use here.
When I saw the title I was assuming this was some cool new sort of client that created and managed a bunch of logins/identities as one on various XMPP servers and used something like OMEMO for key management on top of that 'layer'.
Tox is a better example of a 'decentralized' protocol.
Beside that, Dino does support peer-to-peer file transfers using Jingle.