The argument is not whether they react to stimulus we consider painful, but if they feel it in a way that mature humans understand as pain. Babies take months just to understand that they have hands, they're incredibly simple minded, so the question was whether they even had a real concept of pain beyond "stimulus and instinctual reaction". Similar to how an insect will react to pain but it's just doing what it's programmed to do, there's no higher level thinking associated with it.