Axiomatically, if you sum up over all worldwide assets and liabilities, they sum to zero. That is a fact that simply follows from accounting. So the world as a whole is (net) debt free (until we start trading with extraterrestrials, then things might change).
The US probably is roughly net debt free on average, summing over all domestic private and government assets and liabilities. I don't have time to run the numbers right now. Certainly, even if the US is not debt free on average, its net debt towards foreigners is much less than the government debt, and also less than the sum of government debt held by foreigners.
If people just stopped looking only on one side of the coin, they would realize that and chill out over the government debt.