That's not MMT, though - at least, not exclusively. It's just a truism from what it means to have a fiat currency; essentially nobody will disagree with this, be they monetarist or whatever (obviously they'll disagree about whether doing so is beneficial or not). Most countries can't exploit this because the organization in charge of monetary policy is autonomous from the organization in charge of fiscal policy. You, as the government, need to borrow money, because you don't control the central bank and the central bank is unwilling for whatever reason to simply create new money to give to you. If you want to be able to proscribe MMT solutions (in the sense of "governments should fund short-term stimuli using created money"), you have to argue for either removing the independence of the central bank, severely curtailing that independence, or changing the mandates of the central bank to be strongly different from the status quo.
That's the important conclusion of MMT, and the one that is strongly heterodox - all of the other Keynesian branches (and MMT is a sub-branch of Keynesianism, in the sense that it draws upon the most fundamental Keynesian tenets) think there is more value in having an independent central bank. There's some overlap because there's a growing school of Keynesians who think that inflation targets are actually a pretty poor way for central banks to work, and the nominal GDP targets are actually better, and I suspect this might one day be the new orthodoxy because the early papers are promising and frankly inflation-targeting hasn't worked as well as desired [and being honest, most central banks basically gave up on it altogether anyway during the financial crisis], but most MMT people (not all, but most) are full-throttle "reincorporate the central bank". I don't see that ever happening.