That's not really accurate.

The commercial banks can have accounts at the central bank, and fiat value there is basically "a promise to print physical cash later", but it is "as good as cash", as the issuer of the claim is the same person that creates the physical base money. Thus these central bank reserves are included in calculations of total base money, it has a physical and a digital component.

Creating more base money units does not equal a default on a claim, the fiat cash cannot be redeamed for anything else, it is not a claim, so there cannot be a default.

Sure, printing more base money leads to all types of nasty consequences, it's just economically different from a default on a claim.