“And the US Constitution has a special exemption for all such people born before 1776.”
All those people were citizens of the various states, which was the same thing, since the states formed the nation.
There were no "citizens" prior to 1776. Everyone was a "subject" of King George III.
Even for a long time after 1776, a lot of states kept saying "subject."
This use of "citizen" didn't really catch on until after the Constitutional convention in 1787.
"Citizen", in it's modern meaning, is a Swiss word, borrowed from the Swiss Republic. "Subject" is the English term that was in common usage in America at the time.