You don’t need an account on chatbrainz.org if you already have a matrix account from another server such as matrix.org. You can use your existing account to use the various *Brainz chat rooms.
That is true (and is a great benefit of the Matrix ecosystem)!
However, I currently don’t have a Matrix account on any other homeserver. And I’d imagine there might be others who don’t either.
My main question is: was it intentional to have account registration be disabled for the ChatBrainz server? Nothing in the MB docs, nor anywhere else I could find for that matter, states that the intent is for those wanting to use Matrix to use an account registered on another homeserver.
I think that it was basically set up for the MetaBrainz staff and contractors, and was never open for other account registration. As I recall, the accounts had to be set up manually, although I could be mistaken. @lucifer would know for sure. I set up my account through matrix.org.