I'm assuming the recommendation is coming from the idea that El Salvador might use some borrowed funds to buy bitcoin.

I'm not certain on that but if it is the case, it'll reduce the countries chances of paying back debts in the immediate term (for example, if the price crashes).

Fees would probably be the thing to complain about with bitcoin and not inflation but there's potential el Salvador did this because the imf didn't offer them much in the first place.