It’s true that the 1790 act used the language of “shall be considered as” and was therefore invalid which the Congress recognized as an error and repealed it, but it does illustrate the thinking at the time.
Natural born citizen means born here of citizen parents.
I'd phrase that a bit different. As a legal fiction, it's aim was to naturalize children foreign-born to citizen parent (father). It isn't the use of legal fiction per se that is the problem, it is that this particular legal fiction is constitutionally out of bounds.
If it had said a person so born shall be considered a citizen, no sweat. But it didn't say that. Rephrased, it said a naturalized citizen shall be considered to be a natural born citizen. On paper, that is constitutionally out of bounds.
As a practical matter, the constitution is toilet paper. Our government is in a state of rebellion.
You likely know this, but to give comfort to the public that McCain was eligible, this act was formally claimed in an act of of the US Senate to state a definition, not a legal fiction. What a bunch of schmucks. They treeat us like we are morons.