The author is referring to the exception to the NBC requirement regarding those who were living here when the country was founded.
Initially, until the children who were born of citizens had reached 35 yo, there were no Natural Born Citizens, by definition. How could there be? This passage explains that exception.
It essentially restates the idea that being born in country to citizens is what makes you a NBC, just like the exception does. Otherwise, why would it be necessary?
No, he isn't. The words are simple and straightforward.
He says explicitly that it is not necessary that a man should be born in the country to be a NATURAL BORN CITIZEN.
The "grandfather clause" exception has nothing to do with NATURAL BORN CITIZENS.
So he is clearly not referring to people who were covered by the grandfather clause.
Because such people, BY DEFINITION, WERE NOT natural born citizens.
Initially, until the children who were born of citizens had reached 35 yo, there were no Natural Born Citizens, by definition. How could there be?
We've been over this birther myth several times already as well, although you may have missed it. For an extensive discussion of this myth, see for example here, and here.
It essentially restates the idea that being born in country to citizens is what makes you a NBC, just like the exception does.
Read it again. It clearly says that you don't have to be born in the country to be a natural born citizen. You only have to be a citizen by birth.
Chief Justice John Marshall, legendary Supreme Court Justice Joseph Story, and famed judge and legal expert Chancellor James Kent, as well as "other distinguished jurists," all agreed with this statement that it DOES NOT take birth on US soil plus citizen parents to be a natural born citizen.
Being a CITIZEN BY BIRTH is enough.