Just offering up a slightly different take. It’s more a function of the foreign demand for U$D assets in general. Looking at the current account balance is informative in that regard.
A simple interpretation of this : since the USA runs a trade deficit, it means we are kinda asking the exporters to the USA to “put it on our tab”.
Now the $ denominated assets can be a lot of things. Public debt, private debt, real assets (land & buildings?) or equities.
So not entirely the same as “the future output of the US economy”, but neither is it entirely unrelated to it.