That is very weird. Was it from the family or from an imam? I know breast milk has a few rules around it in islam, but it mostly boils down to sharing breast milk creating a form kinship between the babies sharing the milk. As in, you become in a way sister/brother with the other baby.

But I agree that many Muslims have a...lot of folk beliefs and take a lot of mental shortcuts that lead to reductive interpretations of the religion. In those cases, bringing up good Islamic jurisprudence usually works.