Judges rule on people's intent based on evidence. It is up to the lawyer presenting the statistics to interpret(with testimony from qualified professionals) these stats before the judge in a way that proves their argument. This applies to medicine,technology and many other fields as well. Judges don't need to know medicne to rule on a malpractice lawsuit either.

Judges are required to make decisions in many fields they have not been specifically trained in. If they were experts on every subject matter, there would be no need for lawyers, except maybe to present evidence.

Regardless, the ruling has nothing to do with “statistical fact”. It has to do with whether the policy was intentionally discriminatory.