This study does raise several questions, primarily whether this approach to doing sibling studies is appropriate, and whether studying the issue of breastfeeding over a period of decades when the patterns of breastfeeding were mostly quite minimal is appropriate. But the biggest question is why this got so much press interest.
Why? perhaps because it supports industry's contention that there is no benefit to breastfeeding? The forgiving side of me says, well, we have had so much good news about breastfeeding that anything that disagrees is of note. However, most of me says that there are those who strive to get any bad news out and about as much as possible. For example, this is the first breastfeeding study that is fully covered by the Dairy press, and it is also hyped by certain feminist groups that feel that anything a female body can do is to be dismissed as "biological determinism" rather than as a source of pride and empowerment.
I think that another reason these rare studies get so much press is that we still make it so hard to breastfeed successfully in the US, that folks get a bit energised on both sides. If we had paid maternity leave, which by the way is associated with fewer premature births and more exclusive breastfeeding, there would be little reason for women to have the emotional overlay that we have in the US.
At the Breastfeeding and Feminism International Conference each March, we try to have discourse rather than finger pointing, discussion rather than accusation.
So, until we can help all researchers to look for data with solid definitions of exclusive breastfeeding, and until we have enough data to answer these questions without covering decades of time with all the differences this implies, we will continue to see misleading headlines from the very few studies with negative outcomes.