I came across similar conclusions when researching CA Prop 16, and it makes sense. Many affirmative action-ed students, by virtue of their systematic disadvantage, just can't keep up with their more privileged colleagues. I'm really disappointed, though, that the authors did not address the fact that this "mismatch" is still just another symptom of the problem affirmative action is trying to solve.
If you think these students faced a systematic disadvantage that made them unable to answer multiple choice questions about basic math problems, or material they were supposed to have learned during undergrad, you ought to point out what it actually is, and how big an effect size it has, and why.
Elephant in the room: most of the "privilege" you are talking about is literally parents giving a damn and pouring endless time and energy into teaching stuff to their kids and trying to engrave delayed gratification into their personality.
But instead of rewarding and praising this behavior, and trying to make it a role model for the struggling minorities, we try to punish and discourage the kids that have learned the skills with their parents' help, so they won't succeed in life either.
Of course, this doesn't affect the "admission by donation" kind of families, that leaves me wonder: is it about helping minorities succeed, or is it about replacing arrogant self-reliant challengers with grateful and loyal underperformers, to further entrench the elites' position?