Maybe so, but congress does not have the power to "define" anything in the Constitution as that would be changing or amending and that can only be done by Constitutional amendment.
We have never proposed an amendment to define NBC status although I do think in an article V convention that very issue needs to be addressed.
Defining natural-born citizen is inherent to the act of establishing a uniform rule of naturalization. Congress must determine who does not require naturalization in order to establish rules for those who do.
Similarly, Congress must determine what is legal in order to establish rules for that which is illegal.