Citizenship does not equal Natural Born Citizen. These are entirely different concepts, hence why the Founding Fathers even differentiated the two, clearly setting up a difference between a citizen and a natural born citizen when grandfathering all citizen-Americans for the purpose of the Presidency.
"No Person except a natural born Citizen, or a Citizen of the United States, at the time of the Adoption of this Constitution, shall be eligible to the Office of President; neither shall any Person be eligible to that Office who shall not have attained to the Age of thirty-five Years, and been fourteen Years a Resident..."
The only definition ever in law for NBC is 1790
This was not a definition, but an extension under Congress's power to make rules of naturalization (which, by definition, has nothing to do with Natural Born citizenship!) to treat children of overseas Americans "as if" they were natural born, IOW, a legal fiction, without actually being clear that they are well and truly natural born and eligible to be President. The definition of natural born utilized by the founding fathers according to the principles of natural law required the child to be born under US land and to have parents who are citizens.
To treat somebody as a NBC who isn't a NBC is unconstitutional. You think the Founders were actually doing that?
Congress can of course define rules of Naturalization. But they can't define new rules for being a NBC nor can the make laws to treat somebody like a NBC.
The most likely scenario for the language in the 1790 Act was that they were reiterating the common definition of what it means to be NBC.