There’s sort of the same issue with Matrix (Librem Chat). Chat history is replicated between home servers. I think that pure-Matrix messages might be end-to-end encrypted, with no way for server admins to read their contents.
However, Matrix also very ambitiously tries to bridge various IM islands. Chats, where at least one party uses some unencrypted IM, will of course be sent unencrypted to the (outside only?) world.
And then we have other IM systems that are in fact encrypted, but using different methods. So messages can be almost e2e encrypted, except they need to be re-encrypted along the route. I think that happens on the home server, meaning anyone with access to the server database (or a backup) will be able to read those messages.
But there are many details that I have not fully understood about Matrix operation. Maybe @matthew from the Matrix team can clear things up?