How would they determine that everyone in the house was illegal?
The premise of this article makes no sense.
But for the sake of a scenario let's suppose that a census worker knocks on the door and someone answers. The worker asks the person if he/she is a citizen and if there are more people residing at the house who are citizens. The person answers "no" and the census taker says "Thank you," or "Gracias," records no information about the residents of this house and moves on to the next house.