It seems like the intent is to encourage giving changes to exclusively the maintainer, and that forking in this context refers to distribution beyond a private communication between the change proposer and the maintainer.
Which, this being a git repo, makes perfect sense, as despite the impression GitHub gave to a whole generation of developers, Git was designed with a different contribution workflow in mind: one of sending patches by e-mail. Cloning the repo locally, making changes, formatting a patch, and sending it to the maintainer, seems to be perfectly withing bounds of both intent and text of the license.