This is Simpson's paradox at work. And the article even acknowledges it. Most of the universities on the list have disproportionate numbers of students in STEM fields.

That's why Ivy League schools aren't well represented. And also why places like CMU or University of California don't appear. Additionally, salaries aren't adjusted for cost of living. Most of the universities here are located in areas with a high cost of living, so salaries will typically be higher.