Correct. Dual national at birth, does not make one a natural born citizen of the U.S.
Even if British law did or does not make him a British citizen at birth, the fact that both his parents were not US citizens would disqualify him.
Under the 14th amendment he would be a citizen, same as Wong Kim Ark, but not a natural born on. The 14th amendment does not contain the words "Natural born citizen" or even "natural born". Thus it did not redefine the term as originally understood. Which of course was "born in the country of parents who are it's citizens".