Yes it would have, simply because there is no provision under the law that would have made him lose his citizenship through adotpion.
Sorry, but this is inaccurate. Indonesian law allows an adopted child under the age of 5 to automatically become an Indonesian citizen. Indonesia does not recognize dual citizenship. The U.S. nationality law at the time prevented loss of nationality if the parent applies for naturalization in another country, but as written, Obama did not need to naturalize to become an Indonesian citizen. It would have been automatic. The loss of nationality law also prevents naturalization of the child because the parent naturalizes, but Lolo Soetoro was already an Indonesian citizen and would have had no need to naturalize. Third, the law didn't prevent a minor from disavowing his own U.S. citizenship (which may have been required for Obama). The law was changed in 1986 and is now much more protective than it was in 1965.
I would also add that this idea about loss of citizenship is not a long established principle consistent with the history of this nation, but is instead a product of progressive 20th century statutes. We know this because in WKA it said, "neither he nor his parents acting for him ever renounced his allegiance to the United States," which clearly indicates that it was understood that parents could renounce the allegiance (and thus, the citizenship) of their child. The WKA decision mentions it again at the end of the decision, "Upon the facts agreed in this case, the American citizenship which Wong Kim Ark acquired by birth within the United States has not been lost or taken away by anything happening since his birth. No doubt he might himself, after coming of age, renounce this citizenship ..." This clearly acknowledges that a minor COULD lose his citizenship from an act of the parents, or on his own after coming of age.