The IMF only wants El Salvador to comply with their rules, so any means will always be taken even by leading opinions that make the media confused between believing IMF statements or seeing the fact that El Salvador is increasingly becoming an example for other countries to get out of IMF debt. So don't be surprised if the IMF will not stop here to try to pressure El Salvador, especially in stopping bitcoin buying activities.
A few words is enough for El Salvador to proof it. But to be honest I dont like the idea of forcing a country to comply with them. If El Salvador is to show a prove that shuts the IMF, dont you think other countries that holds Bitcoin in treasury wont bother to comply with them any longer. Because I dont see any good reason they will lie about owning Bitcoin when apart from the bitcoin in the government custody most of their citizens hold Bitcoin.