I find it telling that the article breaks down scores by race, not by income. Note that it claims that the difficulties are for low income families who cannot afford tutors / other resources, not black children. What's the crossover? What's the pass ratio for poor white children?
Granted, a book can easily be harder to read / understand for less-classically-trained children. I'm not saying it's not possible, just that the article is definitely seeing what it was looking for.
The first table cites the pass ratios for all low-income students at 37.3%, 28.3%, and 26.4% for 4th, 7th, and 10th grade respectively -- the pass rates for black children are lower than that. So one can accurately infer that non-black low-income children perform better.
Granted, this isn't as informative as breaking things down by both race and income. But, as far as I can tell, this is a limitation of the state's reporting data and not a selective argument made by the blog.
You have to look at the WASL specifically and filter on other criteria from there. For example, a bar chart of 10th grade scores comparing low/non-low incomes:
Granted, a book can easily be harder to read / understand for less-classically-trained children. I'm not saying it's not possible, just that the article is definitely seeing what it was looking for.