I think the impact of breastfeeding on cognitive development is multi-faceted - it includes some of the ingredients in breastmilk as well as the act of breastfeeding itself (hormonal/physical intimacy, sucking, etc). Babies who are not breastfed will miss out on some or all of these factors.
Yeah that. And perhaps also that the mother is more likely to not be working full time if she is full time BF? That has to have an impact that is difficult to quantify.