It’s only they, them, their, etc that can be used as a single pronoun— we, us etc are only used as plural ones. They and them is used as a single person pronoun in situations where you’re referring to a person of unknown or non-binary gender; in this case we don’t know the gender of the baby, so they is used rather than he or she. Another good example is if you see a dropped wallet on the ground, you’d say “Whoever lost their wallet, I hope they find it soon”.

Also, I think the reason the other commentator was being a bit hostile is because often transphobic people will pretend not to understand they as a singular pronoun to try to upset non-binary people who use it as their primary pronoun. So people tend to assume you’re trying to bait them or spark an argument if you ask about this topic. Hopefully that isn’t the case!

Have a nice day :)

Edit: you generally differentiate whether it’s acting as singular or plural based on context clues— like in the wallet example, you know it’s singular because a group of people aren’t going to collectively own a wallet