How could having more US dollars solidified the 1991 Argentine peg to the US dollar? In an economics textbook, Modern Principles of Economics, the authors, Tyler Cowen and Alex Tabarrok, write that "The Argentine government did not have enough U.S. dollars to keep up the value of the peg,"(referring to the 1991 peg) almost suggesting that had they had enough US dollars that the peg would be kept. How does having more US dollars keep the value of the peso up?

If the exchange rate of the Peso was dropping due to decreased demand(people were converting pesos to dollars), shouldn't the government buy a non-USD currency with pesos to reduce the supply and push the price up, keeping the peg intact. How does having "enough" US dollars solve that problem?

Clearly, looking at this from just one perspective, the exchange rate, is limiting in itself, but the sentence seems to suggests it was a possibility. I would appreciate it if anyone could explain the authors rationale.
Simply having more dollars may have served two purposes. Firstly, more dollars would mean that the central bank could have bought more pesos and exerted some upward pressure on the exchange rate. Secondly, fixed exchange rates can fall because of expectations about the central bank's credibility to its peg. If the prevailing expectation is that the central bank does not have the resources (reserves) or the will to defend the peg, pesos will begin to be sold en masse in the expectation of a devaluation.