https://www.staugustine.com/article/20151216/opinion/312169950
In this article criticizing affirmative action, Sowell discusses minorities failing out of Cornell and MIT. I haven't yet looked at the graduation rate data broken down by race from either of those places... but nor does Sowell cite them in the article. So I'm unable to verify Sowell's assertions just based on what he wrote.
So to help answer that I did my own research and dug up the graduation statistics from MIT and Cornell.
MIT: http://ir.mit.edu/more-student-data
Cornell: https://www.collegetuitioncompare.com/edu/190415/cornell-university/graduation/
The first thing to note is both these institutions' graduation rates are lower overall: About 95% as opposed to the 97-98% of Harvard and Yale. These statistics also show minorities flunking out a bit more often than whites than was the case at Harvard and Yale. Cornell: 87% graduation rate for blacks. MIT: between 80-89%. This particular data fits Sowell's theory better, but the effect is not overwhelming. Some of these minority students may be "mismatched," but the vast majority are not. I'd hardly call it a slam-dunk for the "mismatch" theory.
And anyway, if I'm "cherry-picking" in my selection of my two elite institutions, Harvard and Yale, then Sowell is "cherry-picking" to an equal degree in selecting two other elite institutions, Cornell and MIT.
Sowell also very briefly discusses the authors of the book "Mismatch" and their analysis of the University of California system's experiment with banning affirmative action. That's a whole topic for a different tl;dr comment.