It does give clarity as to John Jays view on citizenship. Everyone that was born in (future) American/had been in (future) America as a citizen of a state, was grandfathered in when the Constitution was signed.
He references Vaughan’s mother as an American. He does not say, “is now an American”.
His mother could not have been an American citizen at the time of his birth. She could be an American citizen on July 4th, 1776.
If Vaughan was an adult on July 4th, 1776 it wouldn’t matter what his parents were. There would be no way for him to receive American citizenship through his parents.
I would think Jay understood that.