Liberty loans were not at the center of a highly interconnected international electronic financial system.
U.S. Treasuries are. Even Argentinian bonds were circulating in open markets.
The US Government is a particulaly soft touch on forgiving loans, that's good for our creditors, but it's not the same as them defaulting on publicly traded bonds.
France radically devalued the franc. I believe this was during the Kennedy admin. Same difference.