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.
It was family elders. They are pretty serious about religion, attend Islamic lectures etc. And they were mostly concerned about pork consumption of donors. No one mentioned anything about forming milk kinship.
With all due respect, the worst people you can ask for religious advice would be family elder, imams in local mosques. In that order. Find someone who actually studied the religion not just someone who memorized a book.
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.