So, being born outside of Britain, you were still British.
If you were born on a boat in international waters, were you still British?
If you were born while your boat was docked in France, were you still British?
The issue of course isn’t whether you were British, it is whether you were considered “natural-born”. We know that there is a birthright citizenship in many countries, and we know that America has long counted as citizens the children of citizens whereever they were born, which allowed people to claim dual citizenship if they chose.
The question is whether, when the founders wrote that clause, they meant to exclude children born to american citizens who happened to be on holiday when they gave birth. Of course, being on Holiday wasn’t nearly as common to them as it is to us. You can easily give birth to a child in Japan, and have the child back in your home in Alabama in a week.
Natural-born citizenship inherently precludes dual citizenship. The Supreme Court affirmed this in Shanks v. Dupont when it said that under the Treaty of 1783, you were either a natural-born subject or a natural-born citizen, including all born in the United States, depending on whether your parents adhered to the crown or whether they adhered to U.S. allegiance. You can’t be both.