> For the first factor, we adopted programmers’ ratings in competitive programming contests (AtCoder), which are objectively determined by the relative positions of their actual performances among thousands of programmers
This makes me extremely skeptical of the study. At best, it says something about people who are whizzes at programming competitions, and certainly not "Expert programmers" in general.
Ever met a normal person... They have no idea what an expert is like. For them, easily ranked = source of expertise.