Relevant quote from the article for the critical info to save some the read:

But the problem with using aeration as a proxy for proof of life, many medical experts argue, is that babies don’t have to take a breath for air to enter their lungs. Air can be introduced when the baby’s chest is compressed as it squeezes through the birth canal. If there is an attempt to resuscitate a stillborn baby, that pressure can inflate the lungs. And if a body has started to decompose, gases from that process can cause the lungs to float in water. Even the ordinary handling of a stillborn baby can allow air to enter the lungs.