The US has been printing money for a decade or so.
Yes but the US feds haven't done so to pay off all of its 20+ trillion in debt. Because they want to limit inflation. Raising taxes by any significant amount to calm inflation would reduce economic activity. So they print money when required when it makes sense, like right now because they have a ton of debt and are in an emergency situation due to COVID when they have to spend a ton of money.
MMT has its points. It says countries with their own currency can't default on their debts, which seems true. They also say countries can print money as long as inflation is controlled, which may be true. I just don't think it's a wise idea to take the theory to the extreme like some people want, or at least be the first country to try it. Japan could be a guinea pig.
There's no telling how sustainable this strategy is, because introducing a lot of money into the economy and keeping interest rates low etc. can create economic bubbles. The role of the fed is to inject money into the economy when it's on a downturn and slow down the economy when things are heating up too much too fast to avoid bubbles.
https://www.usatoday.com/in-depth/money/2020/05/12/coronavirushow-u-s-printing-dollars-save-economy-during-crisis-fed/3038117001/