I would say it would be highly unlikely that a bus driver, no matter how good his use of English, would score higher on teaching knowledge of grammar test. Not because he can't speak as well, or better, than the qualified non-native speaker, but because he may not possess the terminology required.

I have many very well spoken friends who write beautifully... However ask them to explain the difference between present perfect and past simple and why there is a difference and they couldn't.

Which is why for teaching, the one who has completed training and knows how to answer the questions their students will have is better suited than an articulate bus driver.

Quite often, advanced level non-native speakers are more away of the mechanics of a language than native speakers. I know a lot of my FCE+ students speak better English than a lot of English people I know!

There's a lot in what you say LG. However, I did describe my bus driver as having a hobby of[not just writing interesting letters] entering writing competitions. So, it would have been silly of him to have not had a good understanding of grammar, if he entertained any ambitions of winning. From the ordinary folk I have met in life that scenario is not unlikely. I knew an Irish construction worker once with an extremely huge understanding of physics. You'll be surprised what people get up to in their spare time. As for the other job applicant- well, from what I've learned in the past week or so, I'm sorry but it wouldn't surprise me at all if he did so badly against the bus driver. A couple of weeks ago I would have said that you were totally correct.