U Chicago is known for its preference for kids from specific “feeder schools”, to the point that, at least at one private feeder school, admissions to U Chicago was considered to be a “safety”. The fact that any student can consider a college with acceptance rates in the single digits to be a “safety” at any level means that that student has a serious hook. So for U Chicago, attendings some specific “feeder schools” is a serious hook.
So @Medea1’s analysis of the situation seem to be supported by research from UofC people themselves:
https://sociologyhighered.ssd.uchicago.edu/admissions-processes-of-two-elite-uc-feeder-schools/