The submitter is supposed to be the good programmer; if not, then maintainers may or may not review it themselves depending on the importance of the feature.

And yes of course they need to be able to write the code themselves, but that's the easy part: any good developer could write a full production OS by themselves given access to documentation and literature and an enormous amount of time. The problem is the time.

Well, assuming you care about verification, of course. If it's got that green checkmark emoji, it ships!