disappearing
"Breast milk is a bodily fluid, so to feed someone else's baby is akin to having a blood transfusion or similar,"
No it isn't. Completely different things are transmissible through blood and milk. The vast majority of things you could catch from a transfusion you cannot catch from milk.
"this is why cows milk is pasteurised."
No it isn't. Cows milk is pasturised because it has to travel a long way before being drunk, and all milk contains as small amount of bacteria at point of origin. If you wanted to store breast milk for a week while you got it to the shops you would have to pasturise that too, but seeing as it is used straight away, you don't.
"A baby has been inside its own mother before birth, so has already shared all blood/germs/viruses etc. before birth, someone else's baby is obv not in that position."
This is total shit. Unless something went wrong during birth or there is an internal bleed, the baby has not shared blood with the mother, and hence has not shared any blood based viruses. For evidence of this consider that not all babies born to HIV positive mothers are HIV positive. Consider that this is also true if they BF their babies
Apart from that what you said was accurate.....