They did try to control for this. From the article:
(It’s important to note that we controlled for numerous other factors that might influence postgraduation earnings, such as family income, race/ethnicity, gender, marital status, SAT score, postgraduate degree and age at graduation and more.)
Few studies seem to try to control for student attitudinal characteristics which may be described as grit, persistence, perseverance, growth mindset, resilience, internal locus of control...?
This is not to say that the article is not making a valid point. In which case the higher earnings in some fields may be related, in part, to strong alumni networks. This is something which other colleges & universities could potentially focus on building/improving as a means of increasing the future careers of their students.
