According to Article 1,S8 of the US Constitution, Congress was given the authority to establish uniform rules of naturalization. Naturalization is the process by which one who is born a citizen of another country becomes a citizen of the United States. Congress was given no authority to declare as citizens, children born to US citizen parents because that would have been redundant and bad law.
Therefore, since Cruz is a citizen via the rules of naturalization established by Congress since 1940, how does that entitle him to be a natural born citizen where there would have been no authority for Congress to act on his behalf so to make him a citizen?
Nevertheless, Congress has continuously passed laws, since 1790, with provisions for citizenship at birth to those who are born to US citizens and who, depending on the era, meet different other requirements.
They would cite the provision in the Constitution that permits them to pass necessary law that enables them to put into action those powers they’ve been granted by the constitution.