Unbelievable.
It just completely goes against the behavior of any puppy; even a badly bred, badly socialized puppy will be startled by shrill cries. That baby had to have cried LOUDLY if the puppy was biting him.
The only thing I can think of is... the baby was already dead, from whatever, and the puppy played with the baby like he was a toy. And, apparently, police/investigators made a very hasty conclusion that the puppy took the baby's life. Puppy teeth DO hurt, but how could they rip and tear skin, or even cut off breathing, to the point of killing? They would shy away at the first cry of the baby.