I've seen this news elsewhere. But while i don't trust IMF that much, AFAIK they don't have strong reason to lie when they could critic El Salvador directly for pro-Bitcoin policy/action. El Salvador could prove they're not lying by showing purchase history on the exchange, but IMF can't just demand the exchange to hand over purcrase history of account owned by El Salvador government.
IMF has made an accusation and that is their standpoint, it doesn't matter now if people trust their declaration or not it is left for El Salvador, to accept or refute the claim because it is bothering on their national integrity. As it stands now the ball is in the court of El Salvador, to decide the action to take regarding the allegation. I understand that part of the terms of loan between IMF and El Salvador, is that they will not purchase any Bitcoin, perhaps El Salvador, is keeping to the agreement which IMF is trying to make public or they are accumulating through other means. Since the matter is out in the open their is need for El Salvador, to come out and clear the air on their Bitcoin position.