That being said remember if we are talking about the time of the founding then he would be eligible because he would be a citizen at the time of the adoption of the Constitution. At the time of the adoption the NBC requirement did not apply. One need only be a citizen at that time. This is assuming of course the framers would have considered Cruz a citizen due to his mothers status which that itself may even be questionable.
The NBC requirement existing within the Constitution since its ratification.
Except ... he was a Canadian. I mean, who among us can say WHICH of his “birth” citizenships took precedence?
Anyway, he would have had to swear allegiance to the newly formed country on it’s formation to qualify under the grandfather terms. Because in the 1700s the mothers citizenship always mirrored the husbands. In the US.