Since the constitution doesn't define NBC, why not?
It doesn't define "bullets " either, but it certainly protects the right to keep and bear arms, the exercise of which requires "bullets."
Natural Born Citizen is an axiomatic extension of the founding doctrine of the US. It's called "Natural Law", and it was a "thing" in the last half of the 18th century. It was a very big "thing" back then.
Supreme Court does say Congress has the power of naturalization. The law that made Cruz a citizen is an act or statute of naturalization without any action or endeavor from an individual.