Exactly!
To have both British and U.S. Citizenship, or
Russian and U.S. Citizenship, or
Chinese and U.S. Citizenship, etc.
AT birth....would NOT make that person a NBC no matter where that person was born!
If that were the case, the framers of our Constitution would not have had to put "or a citizen of the United States, at the time of the adoption of this Constitution" in Article II section I.
There would be Zero need to do so, if they thought that having British citizenship at birth themselves...but later U.S. citizenship via the Declaration of Independance meant they still meet the NBC clause.
They needed that clause not because they were dual citizens at birth, but because they were only British citizens at birth-- the U.S. didn't exist as a legal entity until 1776.
In addition, ‘subject to the jurisdiction of’ and ‘not subject to any foreign power’ would exclude granting anchor babies citizenship, let alone natural born citizenship.