In my understanding the design doc repo is far more binding than “brainstorm” or “draft”.
I mean, everything what is approved in the design doc repo should really go into the code. Possibly scattered over multiple repos.
So if something comes to a PR and it is implemented along the lines of the design doc, then it should be accepted (design wise); though there is always the option to come to the conclusion that the design needs revision given details found during implementation.
The concept is build around consensus, but to me it’s still a lot more than mere “brainstorm”.
(I don’t have a good overview of open-source processes I have to say)