That’s not at all what the article claims. It presents a hypothetical scenario, based on real SAT average scores, where somebody can reach a very wrong conclusion (teacher unions are bad) based on this data. Then the article proceeds to explain how the difference in the average scores can be attributed to self-selection among students who take the SAT rather than unions. So, the article not only remarks on the suspicious causality but the suspicious causality is the article’s main point.