> The foreign-born child is a citizen AT the moment of birth
Not always.
For example, the Immigration and Nationality Act, Pub. L. No. 82-414 § 301(a)(7), 66 Stat. 163, 236 (1952), the foreign-born child is not a citizen if the citizen parent does not meet the requirements of this statute.
And what circumstances did his mother not meet?
Strongly suggest you review USC 1401 and following.
Secondly, since there is no real question of Cruz’s NBC status, please ask yourself, do you really want to elect a man who hangs his hat on that?