Under the 14th Amendment, if Obama was born in Hawaii, he was a natural-born citizen. Under the Supreme Court's decision in Afroyim v. Rusk, a U.S. citizen does not lose his citizenship by becoming a dual citizen of another country, unless he also voluntarily renounces his U.S. citizenship.
The 14th Amendment has NOTHING to do with the definition of "natural-born citizen" and the phrase does not even appear in the Amendment. The 14th did not define, re-define, clarify, or in any way whatsoever alter the meaning of "natural-born citizen" as used in the main body of the United States Constitution.
There is sufficient evidence that he was not born in Hawaii to warrant discovery if the courts would stop ducking by invoking standing. We don’t know whether or not he voluntarily gave up his U. S. citizenship because he refuses to reveal the information that would reveal whether he did or not and in fact has spent very large sums of money avoiding providing that information. Further, his minions have persisted in using an altered document as “proof” which is generally a tactic that raises an inference of fraud.
But we don't know if Obama was born in the United States and there is enough evidence that he was not to warrant discovery into that question even if the evidence is not itself admissible. Correspondingly we don't know for sure if he in fact ever "acquired" U. S. citizenship. If he never acquired U. S. citizenship then the decision that you cite does not apply. Further, even if he acquired U. S. citizenship, which is very much a question, we don't know if he voluntarily relinquished it or not.