Is the AP CS test supposed to be some sort of impartial arbiter of talent and ability? What does the AP test have to do with anything?

Yes it is - it's a measure of interest in computers and drive/intelligence.

If you don't like AP CS test - pick some other parameter that indicates that this population that should (absent discrimination on part of money-bag holders) produce a number of startup founders.

Most schools don't even offer AP CS. Especially not public schools that primarily serve low-income students (which black people are disproportionately more likely to be). The course/exam is not equally accessible to people across racial demographics, and is, therefore, not a logical metric to measure potential (unless you're maybe looking at percentage of people getting 5s out of the total number of people of that demographic taking it).