Yeah, both the distribution of highly successful people and people with high academic performance are heavily skewed and the slightest decorrelation should result in the phenomenon of heuristically seeming independent.
In an ideally meritocratic world, we should see fewer Harvard grads and magnis cum laudibus occupying the top of the industries.
> the most financially successful people I know had mediocre college grades
is saying something about p(mediocre grades|successful) i.e. among the people I know who are successful, a surprising number had bad grades.
where the interesting question, given the chronology of the two events is, as you point out, p(successful|mediocre grades)