Yes, that's right. Corda RPC is also message queue based. That means you can do things like throttle RPC clients, do load balancing between RPC serving subsystems, persist the response queues etc. All the regular features MQ brokers provide. We don't expose all of them through the RPC API yet but that's the long term plan.
P2P traffic doesn't use the RPC engine because that's more oriented towards async messaging where there can be long gaps between requests and responses, and where there isn't always a clear distinction between client and server.