It's worse than that - the hiring managers were accurately identifying things that matter. The article isn't saying that the hiring managers were unfairly biased, but that they were somehow able to pick up on traits (their level of education, their level of professional success) that they'd otherwise care about.

It'd be more worrying if hiring managers picked up on thing that didn't matter (their parents' level of education, controlled for their own, their parents' income level, controlled for their own, where they grew up, again, controlled for other markers of professional success) but the article isn't saying that's what the study found.

To me it sounds like they were going to come to the same conclusion no matter what. If the hiring managers were unable to identify any meaningful signals and discern better credentials from the recordings, they could've spun this as: the hiring managers, unable to discern things that matter, picked up on irrelevant cues and discriminated against people with certain irrelevant characteristics (it's inevitable that some traits would be correlated with the hiring managers' selection). In this case, the hiring managers managed to discriminate in favor of better candidates - so they are just reframing better credentials as class.