Alito is employing fear mongering to address a constitutional question. Your hypothetical sounds like a bad movie script, and assumes our intelligence apparatus is incompetent.
Are babies of Afrikaner immigrants born in the US this year US citizens? Yes. Of course.
I am more concerned about the brainwashed children born of Trump voters who potentially present as a greater threat than their parents. But, I firmly believe that America will find its way home as a principled and honorable nation, as the bright shining city on the hill that the world reveres.