Yeah, exactly. Babies are too young for gender. It's child abuse to call any baby a boy or a girl. Gender starts developing in the brain at 2-3 years of age.
This is worst response. Others at least said "No, they are babies, they don't even know how to speak, how are they going to be able to know what "gender" even means?", but your "nobody knows she's a girl yet" is so wierd.
Why? A trans person isn't born cis, they're born, as you said, not even knowing what gender is. A trans woman was always a girl, but only someone who knows the future would know that when she's a baby.