I'm sceptical. Strenze's meta-analysis of correlations between IQ and socioeconomic status (operationalized as education, occupational level, or individual income) found no substantial increase in those correlations between 1929 & 2003.

That does reduce my confidence, but only slightly. I think the stratification claim is more specific than what they're testing, but their coarse measure gives an upper bound on how strong the stratification effect could be. (Unfortunately, I don't have the time to delve into this issue.)