We are conflating things here, since the architecture of IRC and Matrix are not the same.
Since due to the federated nature each room will be different - like it is in Jabber.
I am not talking about talking to my friends (since that's already solved with a private irc network), but for random people on rather large public rooms, where everyone will be isolated to their homeserver without manual configuration (granted.. since we have some big homeservers like matrix.org probably they'll able to continue chatting in the room).
Edit: Turns out I misremembered the federation algorithm from the spec - seems like it is indeed currently full-mesh and not a directed graph so I retract my remark.