Did you bother to read the letter? This is directly addressed:

"This may seem like a counterintuitive claim to some, given the widespread understanding that performance on the SAT/ACT is correlated with socioeconomic status. Research indeed shows some correlation, but unfortunately, research also shows correlations hold for just about every other factor admissions officers can consider, including essays, grades, access to advanced coursework (as well as opportunities to actually take notionally available coursework), and letters of recommendations, among others. Meanwhile, research has shown widespread testing can identify subaltern students who would be missed by these other measures."

I'm very aware and don't disagree with you, I'm responding to the comment above this one that said there's no correlation at all.