The article does not point out that sampling quora users might not be (I would say is probably not) an unbiased estimator of the the students of these places as a whole.
From the article:
Also, a word of warning: my dataset was fairly small and users on Quora are almost certainly not representative of their schools as a whole (though I tried to be rigorous with what I had).
I completely missed that, mea culpa, my apologies to the author. I'll leave the message above unedited as a little warning sign to other people who can't read: you too can look silly like me.
From the article:
Also, a word of warning: my dataset was fairly small and users on Quora are almost certainly not representative of their schools as a whole (though I tried to be rigorous with what I had).