I think it's a pretty straightforward thing: intelligence somewhat correlates with life/career outcomes overall, and it's not linear. Separately, IQ tests are reasonably good, though imperfect measures of general intelligence. Also separately, if you look at careers where high intelligence is needed, then IQ correlates much better.

IQ does not principally measure test-taking abilities or SES. Yes, those correlations exist, but their effect sizes are not nearly as large as a certain political ideologies would have you believe. And simultaneously, it's not as ironclad as the other political ideology would have you believe. It's very reliable as these things go, but noisy at the margin.

EDIT: a sibling comment correctly points out that aggregate effects do not always apply individuals.