I think it means that if you created a model that used IQ, quantitative skills, and working hours as inputs and tried to predict pay across all jobs in the US,

I think it means that if you created a model that used IQ, quantitative skills, and working hours as inputs and tried to predict pay across all jobs in the US, it would predict that science jobs should pay higher than they actually do. Which leaves open the question of whether those variables are actually the most predictive of pay levels in non-science jobs; I suspect not.