That’s “throne”, but good point! LOL Bob
It wasn't just to protect this country from someone that had loyalties to another country but to protect other countries from a president making a claim to the throne(how's that) of another country.
It kept the US out of the constant wars that were taking place between countries in Europe when one ruler made a claim to another countries throne based on birth right.
NBC means being born an American citizen and only an American citizen at the time of your birth.
If a person is born in this country to two parents who are not citizens of this country but neither parent can pass citizenship to the child from the country of their origin, then the child would be a NBC of this country.
If the parents cannot pass citizenship to the child, their citizenship becomes irrelevant as to the child's citizenship.