I don't know what bothers IMF, because whether they've made a purchase or not is up to them. The IMF tries to support a nation in its well-being, but these days such incidents aren't happening. In the past, the IMF had big roles, and now countries are trying to stay self-reliant and improve their economy. After the young president of Burkina Faso made the country people work for the betterment of the nation, there was a big transformation. Similar things were experienced by El Salvador on the adoption of Bitcoin. Even if the country is lying to the world for the betterment of the nation, there's nothing wrong.
Since El Salvador is borrowing money from the IMF and as a creditor, the IMF is not happy and comfortable with the debtor using that money to invest in assets that they consider too risky.
The IMF would not be able or would stop intervening immediately if El Salvador stopped borrowing money from them but I doubt that El Salvador would dare to do so as their economy is on the brink of default. Bitcoin cannot help them improve their economy as expected, and now other than borrowing money from the IMF, they have no other solution to save their economy.