The idea is that if you believe that humans have no innate differences and you observe group differences, then you must attribute these group differences to external discrimination, and hence injustice.
If you accept that groups of humans might have innate differences, you are no longer required to attribute group differences to discrimination. Some group differences might arise from discrimination, but it's plausible that they might not.
A lot of people take it as an article of faith that there are no human group differences, so they're forced to look for discrimination in order to explain the world. For these people, no amount of evidence can prove the fairness of the CS world, since the idea that (gender disparity -> discrimination) is a necessary logical consequence of their worldview.