If you click through to read the underlying research you get a massive jumble of conflicted studies, including several meta-analyses that fail to find any significant results.
This gets reduced to "breastfeeding lowers the risk of breast cancer" in parenting lit without any context about how radically uncertain this area is.
Becoming a parent and actually reading the underlying lit has nearly destroyed my faith in medical research.
Of course we can all agree that in ideal circumstances, breast milk is the best thing for babies, its unique biologically adapted (even between feeds) formula is impossible to mass manufacture and biologicals like leukocytes are impossible to add in.
However, if the mother does not produce enough milk, then failure to thrive concerns take over, supplement.
If mother is post partum, dad can take over and supplement.
People just eat the tagline these days instead of doing a basic cost/benefit analysis and thinking a little bit for themselves.
Add to that the seeming acceptance of bad science and you end up with anti-vaxxers.