No she could’ve renounced her citizenship before he was born, or maybe just failed to register his birth in the US.
Behind the Blue Wall,
Cruz’s mother wasn’t in Canada long enough before Ted was born to qualify for Canadian citizenship.
Well, that didn't happen, and it would have had to be accepted by our government to actually mean anything, legally.
"failed to register his birth in the US"
He still would be a citizen. At best, he would have to file a single form to get the government to certify his pre-existing US citizenship.
That is USCIS Form 600, whose instructions read, in part (emphasis mine):
"Citizenship law has changed over the years and different laws apply to determine whether you automatically became a U.S. citizen at birth, or after birth but before you turned 18 years of age. If you are claiming U.S. citizenship based on your birth abroad to a U.S. citizen parent(s), the law in effect on the date of your birth applies. For purposes of these provisions, you must be the biological child of your U.S. citizen parent, and different provisions apply depending on whether you were born in wedlock or out of wedlock."
The bolded parts demonstrate quite clearly that the citizenship of someone who hadn't been registered as a child is pre-existing, even if the government has never certified it.