I don't think the ATAR alone is a good measurement of whether somebody can teach, however, I think it's a combination of two things:
1) A value for education as a whole and wanting the world to become more educated
Before we even talk about knowledge, we should talk about the valuing of education. Generally, you are right, students with high ATARs will care more about education than students with low ATARs, thus, high achievers might be better teachers, however, it might not be so. I know students who have gotten low ATARs not because they wanted to, but because of their circumstances. When those students then go on to university and learn more about their subject field, they become better at it because they essentially value education. I have also seen students with high ATARs dropping off to satisfactory university marks because they no longer have the motivation.
2) The ability to communicate and teach
This is important as well, I know a lot of amazing minds who cannot teach even if their life depended on it
