I mean it in a practical sense. Let's say you want to contribute to BTCPay Server. They run their own mattermost service. Then you work for another project or a company and there you have a different server. Then you have an organization you're helping. Suddenly you have 5 tabs with beeping chat apps, different identities, different groups of private messages.

I don't mind centralized coordinator, even the matrix rooms are hosted on some home server that takes care of the membership.

What matrix does nicely is integrate all this mess. They even have bridges.