From a pure accounting point of view they are using the money borrowed from the IMF to buy bitcoin, although in practice this is not the case, as the bitcoin is being bought from one part of the budget while the IMF loan is being used to finance other items.
How much help does this have for the El Salvadoran economy?
Not much at least for the moment, as the bitcoin they hold is worth $600 million while their GDP is $34 billion. But if they continue to buy and bitcoin continues to appreciate, there will come a time when it will be very important for their economy.
It's a good start, and it all starts little by little: in the long run, they will get the most value of what they do now..