Intelligence is strongly genetic. Like 95%. And several % from being breast-fed.
Howver you can *reduce* effective IQ with bad nuture. Meaning - bad food and drink, not so much bad education.
For completeness - it’s also possible that the IQ tests used in this case introduced artefacts.
I disagree with the breast milk part.
Only that some women are not able to produce good milk. Having a supplement will improve the outcome of the kids in the case where a Mom isn’t able to supply their dietary needs.
Our kids were in that boat, and we had a new doc try to guilt us into breast feeding when our oldest just wasn’t getting enough. An older doc called and said to use formula to help fill in the gaps. Kid went for lethargic to happy in one feeding.