That is exactly why the Founders grandfathered themselves.
(“or a Citizen of the United States, at the time of the Adoption of this Constitution”)
This Grandfather Clause of Article II applied to the qualifications of 1) Washington, 2) Adams, 3) Jefferson, 4) Madison, 5) Monroe, 6) J.Q. Adams, 7) Jackson, and 9) Harrison, all of whom were born on British Soil, and to parents who were British subjects.
The first President not qualifying under the Grandfather Clause, but as a natural born citizen, is 8.) Martin Van Buren. Van Buren was born on Dec. 5, 1782, near Albany, NY, to two parents who were both U.S. Citizens.
Every predecessor to Van Buren was born British Subjects and with British allegiance through the British subjecthood of their parents.