The only reasonable way to avoid backdoors is to meticulously check the submitted code.

Which is the job of maintainers. Which now aren't Russian, any more. To the best of my knowledge the kernel is still accepting code from Russian citizens, ultimately not having Russians in maintainer roles isn't going to stop the FSB from infiltrating the kernel but it certainly does make it harder.

This also isn't in any way a judgement on the removed people, it's just that it so happens that if you're a Russian citizen you're quite vulnerable to wrench attacks. You could even say that the kernel org is protecting them from being used like that.