I don't know how common it is to cross the border illegally to do it, but foreign nationals do come to the US specifically for the purpose of having children. It's called "birth tourism": https://en.wikipedia.org/wiki/Birth_tourism#Birth_(and_abort...

A Chinese woman could have additional reason to do so as a means of circumventing China's "two child" policy.

Again - to be clear, I have no idea how common this is in practice, much less how common it is for Chinese nationals to enter the US illegally for this purpose or if it's applicable to the people in the article.