I'm sure they ruled out the more obvious confounding variables. All else being equal, you would expect a result like they got, based on the null hypothesis that

I'm sure they ruled out the more obvious confounding variables. All else being equal, you would expect a result like they got, based on the null hypothesis that expensive CEOs are at least average among CEOs, combined with the suspicion that their hiring is not a random event. Factor in that the researchers were undoubtedly biased in favor of the outcome they found, and a lot of skepticism is justified in making sense of this.