The crux of the biscuit is in the swap. The terms aren't clear yet, but the currencies must be swapped back (likely with interest on both the US and argentina's part).

I don't mean to defend this fiat swap (or argue with your pemise as I also wish they'd start purchasing bitcoin) I just thought I'd chime in to clarify that it's not a direct 'purchase' ... the US won't be able to use or rehypothecate the pesos and they will be paid back (possibly with interest) at a specified time.

There are geo-political reasons to extend this offer as well, like combating China's influence in the Americas, stabilize market production of rare earth's, etc.