No one has refuted this yet. A central MMT claim (and post-Keynesian claim, more generally) is that a govt which issues its own floating, non-convertible currency cannot be forced into insolvency on obligations denominated in that currency. This MMT (and post-K) claim is important because it changes how we assess deficits, debt, and spending constraints. Although the govt can experience inflation, depreciation, and resource shortages, those are different from an involuntary default on obligations because it is unable to obtain its own currency. If you can demonstrate that such a govt, willing and legally authorised to pay, nevertheless cannot execute payment because it has exhausted its financial capacity, then that would refute the claim.