Due to the nature of this forum and this question, I was speaking in terms of privacy/security. It is due to privacy and security issues that both XMPP and other Matrix clients were never recommended by Privacy Guides. Degrees of decentralization is off-topic here (unless OP clarifies that they’re specifically looking for that) and we’ve already discussed the issues with federation separately in the past. If I recall correctly, my primary recommendation (SimpleX Chat) is decentralized (perhaps even more so than XMPP?) without being federated.
It has many other issues, such as centralizing power to servers (like all federated platforms) and suffering from abysmal privacy/security flaws, which is what is most relevant here.