Infants should not really be drinking anything else besides breast milk or special milk in general. Like not even water... So I could see how militant vegan people could endanger their babys. But not because plant based milk is inherently bad.
Just because in your socio-economic circles, no one is a militant enough vegan to feed a nursing baby Oatly or whatever, doesn't mean people out there don't need some guidance.
I mean there's vegans out there putting their carnivore pets like cats on vegan diets, so you can see someone thinking "hey it's just another milk replacement.. it's healthy!" feeding it to an infant is certainly not a 0.001% edge case.