I don't question the study's results... but I question exactly what IQ measures, especially in the range below 100. Having taught in high school for several years, I would tend to think that in addition to 'intelligence' what is being tested is reading comprehension and knowledge base. Those things are largely derived from parental values. Kids from homes where an education is valued do better; that is they read and comprehend better, and they have a larger factual base to draw from. As a result, they would score higher on most any test. In my experience, Jews and Asians place the highest value on education so they tend to score the highest and do better. it is not generic, but environmental. Blacks and Hispanics, as a rule, do not place the same emphasis on education.
Yes, i know there are exceptions... I know numerous blacks and Hispanics who are high achievers. Most have parents who understand the benefits of education. A few have parents who don't, but the kids are smart enough to see 'a better way'.