I assume the author thinks that would be a good proxy for some hard-to-measure quality, and is not actually aiming to hire the best typists because of their typ

I assume the author thinks that would be a good proxy for some hard-to-measure quality, and is not actually aiming to hire the best typists because of their typing skills. As for what that quality is, I don't see any hints there. It could be anything from general tech-mindedness to even basic diligence or who knows what.