Isolating this variable allows one to study how good hiring managers are at estimating base probabilities for education/competence given social class (news alert: they are not independent).

It says nothing about what actually matters: do hiring managers with the ability to properly interview "equally qualified" candidates from different social backgrounds do so effectively (i.e. offer them similar compensation), or do they still rely on the base rates (i.e. bias), and how big is that effect?