College Board, a for-profit private corporation, is the company responsible for administering the SAT/ACT which is essentially a college entrance exam. Its cost is non-negligible for a low income student and it can be prepared for with classes that once again have a cost that is non-negligible.
As the article states, colleges also have a profit motive and want to maximize their revenue. They are not incentivized to admit high-performing, low income students when they have a low probability of having their families donate prior to admission and just as likely have a low probability to donate after graduation. Remember that upward mobility is not just a factor of alma mater but also your network of high worth individuals that allows you to have the disposable income to donate.
So it's money all the way down. We can keep kicking the can further down the road and blame the next party. However I think it's worth stopping this issue with the employer. Outside of credit risk, the income of a potential employee is irrelevant if that individual is capable of handling the responsibilities of the position.
Yes, alma mater is a strong indicator of success and capability. However so is self-directed learning, examples of big fish in the small pond, etc. We need to further democratize free/low-cost education by investing in that infrastructure and content at the community level. Most importantly, we need to remove the stigma of not having a shiny diploma.
Employers need to also invest beyond the current employee and into communities to get these potential stars out of the vicious cycle of inequality.