Or are you contending that a citizen necessarily encompasses “naturalized citizen” as well as “natural born citizen” EXCEPT at the time of adoption?
The term "citizen" necessarily encompasses "naturalized citizen" and "natural born citizen" first because that's how language works, and second because it's in the plain language of the 14th Amendment. "Citizen" also applied to both categories at the time of adoption, which is why the Framers carved out a specific one-time exception to the Natural Born Citizen rule.