Survivorship bias occurs when researchers focus on individuals, groups, or cases that have passed some sort of selection process while ignoring those who did no

Survivorship bias occurs when researchers focus on individuals, groups, or cases that have passed some sort of selection process while ignoring those who did not. The author made the case that history's geniuses practically all had private tutoring and that the lack of private tutoring may explain today's lack of apparent geniuses. What was the selection process that he ignored? And how does the lack of that selection process recognition hurt the author's argument?