That was a naturalization act that was revoked. Right?
Whatever it was, it is said to define NBC. If you were in the class of "children of citizens of the United States that may be born beyond Sea, or out of the limits of the United States," you are NBC. If you are not in that class, you are not NBC. That's the way definitions usually work.
People who see the Act as a definition (and I would say that is the vast majority of them who confront this) reject the notion that this part of the act created a legal fiction, and was not a definition. But if it was a definition, it would have included at least some of the people born in the US (not Indians, some reference to state citizenship, and so on).
Seeing as how the 1790 Act doesn't define the citizenship of persons born in the US, I wonder where the people who cite 1790 Act as a definition go, to find the source of citizenship of persons born in the US, before the 14th amendment existed.