It could also be a question of cultural competence. Higher income people tend to have more education so they know how the educational system works. Thus they can position their kids to do well on the tests, as was suggested by the article.
A good example of this is jigsaw puzzles. Jigsaw puzzles were one of the first tests used to measure IQ. In response, middle class parents bought more jigsaw puzzles. Their kid’s IQ increased. So, does exposing children raise their IQ or does it just help in passing a test? If it is just the latter than there are issues that need to be addressed.
On a side note, the article made me chuckle a little. I went to a magnet school in Houston many years ago. Back then its criteria was explicitly racist. Houston was under a court order to integrate. All of the white rich kids went to private school leaving behind poor black kids, so there was nobody left to integrate. The magnet schools were created to lure white children into the system so some balance could be achieved.