“The phrase ‘shall be considered as’ operates to create a legal fiction.”
There are at least two serious flaws with your explanation. One, the phrase is used in that act to describe natural born citizens AND naturalized citizens. It is used three times:
The first is applied to those specifically going through the a procedure to become citizens. The law specifically calls this first group naturalized. The second is applied to the children of the first group who are automatically conveyed citizenship without any additional procedure. (This would apply, for example, to a baby of an immigrant who just became a citizen by an oath of allegiance because the baby cannot take the oath.) So is it “legal fiction” when applied to their being naturalized?
The second flaw with your argument is that you are saying that this law, enacted just 18 months after ratifying the Constitution, was unconstitutional. You actually called it “legal fiction”. That is just a polite way to say you think the founders who ratified the Constitution did not know what they were doing when they passed this law, or worse they were in rebellion against the authority of the very document that formed this nation. Makes no sense.
“Shall be considered as natural born citizens is a proviso.” Alexander Porter Morse, Treatise on Citizenship
If the act is merely a definition, that would be the case, would it not?