The value of physicians' underlying human capital is estimated by forecasting an age-earnings profile for doctors based on the characteristics in youth of NLSY cohort participants who subsequently became doctors. Published estimates are used to measure the total cost (wherever paid) of investments in physician training. These data are combined to compute the societal cost per primary care physician visit. The estimated societal cost per primary care physician visit is much higher than the average co-payment per primary care service and generally higher than the current Medicare compensation rate per service unit
What I think they are saying is that Americans with the talent to be doctors but who choose other careers do at least as well financially, if not better, than those who become primary care doctors.
They are certainly going about answering the question in a more economically sensible way than just looking at doctor salaries compared to average salaries and contrasting this with other countries.