He was born a US citizen though. The requirement isn't that you are born on US soil, but that you are born a US citizen. John McCain was born in Panama.
McCain is distinguishable from Cruz' case though. He was born on a Naval Air Base in a US territory (the Canal Zone) which is legally a lot closer to US soil than Canada is--and some of the old common law precedents specifically address cases like this.
technically it says you must be a 'natural born' us citizen. who the hell knows what that means. If we assume the framers were well-read and fans of shakespeare, this means that anyone born by caesarean section is disqualified from holding the highest office. They were wise enough to know that the dermal microbiome of c-section children is destabilizing creates individuals of temperament ill-suited for the role.
The requirement is that you be a "natural born" citizen.
That said, although legal scholars can have how many angels can dance on the head of a pin debates about this, the most reasonable interpretation is probably that they're US citizens/eligible for US citizenship from birth. If push ever came to shove on this question, one would hope the courts would side with the most liberal interpretation.