I don't get why he doesn't like Pull Requests on GitHub. It obviously depends on the kind of mailing list software you use, but most of them (like shudder GNU Mailman) strongly discourage the user to hunt for existing requests and make pretty much everything hard to follow except if you follow everything all the time. Of course, if you're a maintainer, it's your job to follow almost everything all the time. But casual contributors simply don't have that kind of time and energy available.

With PRs on GitHub, a corresponding issue is usually filed and if people still email you, simply tell them to first search there and then post their own. That way, everybody can see and search the existing stack quickly.

I recently tried to contribute to a project that maintains both a mailman mailing list (I offer a thousand internets to somebody who builds a general purpose and usable UI to that) and a GitHub tracker and it was an absolute nightmare to coordinate.