Because if you print your own currency, the risk of you running out of it (insolvency) is exactly zero.
Of course, when you thought you heard "major" what he probably said was "sovereign in the currency they borrow". Big difference (US is sovereign; Japan is sovereign; Greece is not; Zimbabwe was not).
"and could lean on their massive power/influence if they got stuck"
"Stuck"?
In that situation, our balance sheet would be ok, but our country would be in very bad straits. So we would be stuck in a real sense, if not in an economic sense.
His lecturer probably (naively) believes that America will be dominant for so long that we won't have to worry about pesky things like the threat of war from other countries on economic grounds.
Why? The US government could pay off all of its debts tomorrow by printing money but the hypothetical situation where it would 'need' to simply won't arise.
Same for Japan. Or the UK. Or anybody. Inflation is the only real problem most governments need to concern themselves with (and is inflation too high in the US or Japan?).
>His lecturer probably (naively) believes that America will be dominant for so long
And I suppose Japan has managed to remain solvent with ~220% debt/gdp by being a superpower as well?
His lecturer wasn't being naive. The austerian economists who predicted hyperinflation in Japan in the late 90s for the exact same reasons you're proposing were being naive.
It could arise pretty easily if international markets lost confidence in the USG's ability to repay. In terms of wealth, I mean. Of course the government can print money to pay in dollars, but from an investor's perspective there's no difference between a country that inflates its currency and a country that only pays a percentage of obligations in an uninflated currency.
Most US debt is short term. It has to be rolled over, and it has to be rolled over at very low interest rates. That was the real genius of QE - they found a way to print money and lower interest rates at the same time.
>And I suppose Japan has managed to remain solvent with ~220% debt/gdp by being a superpower as well?
Japan has remained solvent because that 220% number isn't real. If you subtract out interagency borrowing it's more like 130% (or it was, anyway, the last time I looked into it), which, while not ideal, doesn't exactly put the country in a league of its own.