It has nothing to do with US law. Indonesian law stated that he had to choose upon reaching 18. He could not retain Indonesian citizemship without renouncing his US citizenship.
It has nothing to do with US law.It has everything to do with US law. Indonesia does not decide who is a citizen of the US. See the Hague Convention of 1930. Signed and Ratified. Valid, binding law in the United States.
Article 2Any question as to whether a person possesses the nationality of a particular State shall be determined in accordance with the law of that State.