My take is people question the tests because wealthy/powerful people have a history of "gaming the system" in order to retain that wealth/power. It's not unreasonable to ask "is the test predictive of anything other than the parent's ability to hire a tutor?" While a test is, in theory, easy for a poor student to pass through independent studies/library, etc, reality gets in the way - that student needs transport to the library, time to study (many work), etc.

But, based on this research, it seems the answer is "Yes, the test is actually predictive of something other than parent's SES."