It seems incongruous to say that the singular word ‘citizen’ applies only to such persons at the time the Constitution was adopted while the same singular word ‘citizen’ appears in what apparently is the present day Constitution as to eligibility for congresspersons. I grant that words can be meant to be anything desired but events of history bound by words is more than just someones desire/intentions.
For example one must be a “citizen” to be a U.S. Senator. We have had U.S. Senators who were citizens at the time of adoption. We have had Senators who were natural born citizens. We have had Senators who were naturalized as U.S. citizens. We never have had a non-citizen of the USA as a U.S. Senator - and THAT is what is forbidden by this language.
Natural born citizens are “citizens”, naturalized citizens are “citizens”, and those who were citizens at the time of the adoption of the Constitution are ALSO “citizens”.
So who claimed that “citizens” only applies to such persons who were citizens at the time of the adoption of the Constitution? That is apparently a strawman of your own invention.