That’s a very good point. I hadn’t considered potential lack of domain knowledge at all. In that case Rust might even help, because it’s easier to write interfaces that can’t be used wrong - so that even someone without the needed domain knowledge might be able to fix compile issues without breakage.
I would be very surprised if they wouldn’t fix all 50 filesystems.
In all projects I have worked on (which does not include the Linux kernel) submitting a merge request with changes that don’t compile is an absolute no-go. What happens there is, that the CI pipeline runs, fails, and instead of a code review the person submitting the MR gets a note that their CI run failed, and they should fix it before re-opening the MR.