I agree with half of this argument but think you're taking it too far when you reject IQ testing as an epidemiological tool. It's not dispositive. It's statistically ill fitted as a ranking tool, for the reasons you stated, and the fact that it's not reliable across its whole range, and test-test reliability issues, &c. But like, "you can use it as an indicator for environmental causes of cognitive harm" is actually a valid argument, I think?