Grandfathering became real in 1788. This letter was written in 1781.
How exactly does that work?
Well, look at the language of the Constitution. There was a clear distinction between citizen and Natural Born Citizen. There was, regrettably, no clear definition written into the constitution as to the difference between the two. However, as two are mentioned, two there are.
You are welcome to define each, site your sources, just as you should site your source for grandfathering re:American citizenship not beginning until the year 1788.
It is a conversation worth having.