In terms of math skills, sure. But a senior in high school should have enough mastery of the material to explain things to a 9th grader, so that's a bad argument. As anyone who's taken an undergraduate class in the hard sciences knows, a PhD does not indicate anything about teaching ability (in my experience, it's anti-correlated with it if anything).

I'm sure the credentialing process has its own unique failings, but it is trying to act as a real value add.