Their "intelligence" metric was simple IQ, and the average management IQ of their sample was 111, which isn't exactly genius.
Managers need a degree of social intelligence that won't necessarily show up in a basic IQ measurement.
Without looking at the study (so they might have taken this into consideration), but there might be a bias such that intelligent people rise to leadership position despite lacking leadership skills.
I guess it's a good thing that the tie cuts off blood supply to the brain.