A “uniform rule” applies to all, citizen and non-citizen.
Under some conditions a citizen can lose their citizenship (title 8 section 1481) even if they are a NBC, and may wish to re-establish citizenship later. They would need to be naturalized.
The “positive law” or Constitutional law can DEFINE who is not a citizen and requires naturalization AS WELL AS, defining those who are citizens and do not need naturalization.
To take this to an extreme hyperbolic example, Congress could pass an act that says that every person born, regardless of location or parentage, who is born with blue eyes, shall be a natural born citizen of the United States. Because of Article I section 8 clause 4, they would be an NBC.
Okay, then where in the Constitution is a citizen defined?