The data is from NSF’s Survey of Earned Doctorates. According to the data from the survey, the number of students from UChicago who received doctorates in the decade of 2010-2019 is around 1,800. Projecting backwards, that would represent around 13,000 students who graduated. That is 13.8%.
The number that I used were a summary since 1958, and PhD production has increased in the past 20 years. However, it has also increased in the other colleges presented, so the relative production is still the same.
So, for example, in those same 10 years, roughly 1,700 Princeton graduates earned PhDs, and this represented 10,000 student who graduated, so around 17% who earned PhDs.
In any case, the Chicago numbers are still lower than the 15%-20% claimed by U Chicago.