The study apparently used some flavor of IQ test, which already makes assumptions about what “smart” means.

And is who “really smart then”? I knew plenty of people in college who were of similar general IQ as me but chose career paths specifically to maximize income (finance, largely). And I knew plenty who were smarter than me who went into fields such as education or PT/exercise science because that’s what they wanted to do.

So it doesn’t surprise me in the least that IQ correlates with high income to a point, but beyond that point other factors (largely desire to be wealthy) play a larger role.