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.
To retain his Indonesian passport he would have to renounce his US citizenship. THAT was Indonesian law at the time. IF he renewed his Indonesian passport, he could only do so by renouncing his US citizenship. WHAT is so hard to understand about that? It has nothing to do with the US as Indonesia is a sovereign country. The US does not control other countries citizenship laws.