We have to read the sentence very carefully. It's saying that regardless of socioeconomic factors, the number correlates with graduate success rate. This seems like a very easy "duh". The way I read that is "if a student gets in the 99th percentile regardless of whether they grow up rich or poor, they are likely to do well at MIT". This doesn't talk about acceptance rates based on socioeconomic factors.

The point in question is whether the students in a lower socioeconomic situation even has a chance to get into MIT.