You're shifting the goalposts. Lack of protection against invasion doesn't mean you're going to get invaded. Maintaining a standing army isn't protection against invasion either.
Do you agree that if a nation is successfully invaded its currency will lose its integrity?
Do you agree that defense against invasion requires an adequate military or agreements with nations with adequate militaries?
If we agree on those two claims, it seems we must agree that militarily robust sovereignty is necessary but not sufficient for the integrity of a fiat currency.