The point is that the definition of natural-born citizen is based on statutory law because it is not defined in the constitution. And Tribe knows that based on the statute that applies to Cruz, he is a natural-born citizen.
Citizenship is defined in statutes; natural born citizenship is not (except in one statute passed 225 years ago, that was repealed five years later).
Clause 5 of of the 14th amendment does not grant congress the power to "substantively define or interpret Fourteenth Amendment rights". It most certainly doesn't give it the right to establish definitions that substantively define or interpret aspects of the constitution completely unrelated to to amendment 14.
It is also worth noting that the 14th amendment defines persons "naturalized in the United States, and subject to the jurisdiction thereof" as being CITIZENS.
As the constitution has previously instructed us that there are CITIZENS and there are NATURAL BORN CITIZENS, the 14th amendment has specifically stated that any person naturalized is a simple citizen and, therefore, not eligible to be POTUS.
Title 8 laws gain their authority through this 14th amendment. Nowhere does the 14th amendment make mention of granting anyone the status of natural born citizen but, in it's first five words, it does indicate to us that there are such things as natural born citizens and they are distinct from citizens not born here.
Furthermore, we have the 12th amendment, written by the 8th congress that further establishes that it was the intent of the Constitution's authors to preclude the office of Presidency to non-native born citizens by stating that no person constitutionally ineligible to the office of President shall be eligible to that of Vice-President of the United States.