you are not totally wrong with your assumption because sometimes, when it is done prior secretion time of the hormone into the blood circulation, it will return as false negative.
But in their case, she had already conceived unknowing to her. They came to come only when she realized she has missed her monthly menstrual flow.
To be specific, she came after about three weeks post sexual intercourse.
Ah OK. Then it was definitely not too early (I just didn't have that piece of information ;) ). Thanks for the clarifications, and have a nice week-end!