You're confusing mechanism with economics.
"China isn't the US's lender and they do not make any loans."
Yes, China is a lender to the US, in fact the largest lender [1]. Treasures are loans/bonds. Whoever owns them is a lender to the United States. It's economically the same thing as any other nation selling bonds.
"They can't foreclose, because no loan exists. All they can do is take their coupons or sell the security to someone else who will do the same."
It's the same thing as government debt in all other nations. If you loan money to Greece, you can sell that Greek debt to someone else. The economics of government debt are the same in the US, Canada, and US even if 'how it gets sold' is different.
"China can't starve the Treasury of money "
China can 'starve' any nation to the extent they are a lender to that nation. China's demand for Treasuries makes up part of the demand curve for Treasuries, along with all the the demand. In just the same way as there is demand for Greek, UK and Canadian bonds. Or stocks. Or corn, or whatever.
If China was the only buyer of US Treasuries you can dam well be sure they can 'starve the Treasury' - because if nobody is buying Treasuries, then the government is printing money.
"Also worth noting is that with the stroke of a pen Congress and the President acting in concert could restore the Treasury's power to spend money into existence directly."
Every nation on Earth can do this. It's called 'printing money'. It's not a new idea, and it has dramatic consequences including hyperinflation.
Summary:
China is the #1 lender to the US both in Treasuries (and by holding US dollars.)
US government debt operates differently, but economically is the same thing as government debt to basically any other nation.
The US does have the advantage of 'seignorage' which is to say that because so many people need US dollars to do so many things, that there is a nice bit of 'cushion' in demand for USD, but that's only worth so much.
Otherwise it's just like anywhere else: governments loan at a certain rate, or they can tax, or they can print money and see confidence lost in their currency.
[1] https://en.wikipedia.org/wiki/National_debt_of_the_United_St...