MessageBus and Redis serve very different purposes.
Redis is an in-memory database, used to store state on the server that is more or less volatile. Some examples are scheduled jobs, session data, rate limiting data, …
MessageBus is a system that is used to push messages to the client. For example, if you’re on Discourse while someone replies here, you’ll see a blue number appear an your avatar immediately – that piece of information was sent from the server to your browser through MessageBus.