I believe your quote is from one of my answers!
Yes, there are contexts where you can call a baby, or a person a "thing". In British English "poor thing" is a common expression when feeling sorry for someone.
However, in the context of my answer, I wasn't referring to the baby as a "thing". I was referring to a quality or characteristic of the person.
A person is many things - a man, a woman, tall, short, kind etc. All these qualities could be called "things". For example, it is quite idiomatic to say "tell me something about yourself", to mean a fact, or a quality, or an aspect of your personality.
I said that the baby (who is a person) is "not yet anything more than a baby with respect to age, growth, intellectual capacity etc". I was referring to the fact they are a baby as a quality of the person rather than it meaning the entire person themselves. One day that person will be an adult, not a baby, so they will lose that characteristic, but they will still be a person.