I choose to interpret this data as suggesting that while large changes in IQ may have practically significant effects on academic performance, a change in 3 IQ points certainly does not.
This gets to my point. What makes me nuts about studies like this is that they confuse statistical significance with practical significance. Statistically, 3 IQ points may indeed be significant, but will it tell you anything about any particular set of siblings? No.
Here is my test for practical significance… If I am walking down the street and see someone, I have a limited amount of information about them. From that limited information, I am forced to draw conclusions. Would knowing that they are the eldest sibling let me infer that they are smart? No. Would knowing that they are the eldest sibling even let me infer that they are smarter than their younger siblings? On average yes, but average isn’t really going to help me here. The distributions of intelligence between the eldest and their younger siblings overlap so substantially that no reasonable statement can be made about a random person met on the street.
This is the same reason why racism and sexism are useless. (We will put aside the moral issues.) They are useless because race and gender are not good proxies by which I could infer meaningful statements about particular individuals; therefore, I should not base my behavior towards them on those statements. Prejudice is not only ethically wrong; it is patently illogical.