Writing code that meets code-style expectations isn't just arbitrary, though - it's exactly what your brilliant contributor will need to get in the habit of, if

Writing code that meets code-style expectations isn't just arbitrary, though - it's exactly what your brilliant contributor will need to get in the habit of, if he/she wants to become a brilliant maintainer in the future. As you mentioned, the form of communications matters here: A friendly bot that describes where linting failed and what needs to be done to pass CI, plus a brief "Thanks, this feature looks really interesting! Once it passes Danger I'm happy to merge!" from the maintainer will do the trick. There's no need for the core maintainer to be a janitor at all, just an encouraging voice. And that's what we love about open source in the first place, right?