Is that when you buy US Treasuries, you don't actually get anything. They don't send you a magic stone with powers to call in a debt. What happens is there's an entry made in a computer database, a computer that is in the US.
What this means is that the US ultimately has control over the repayment. Now both legally and practically the US is obligated to repay their securities per the agreed upon terms. However, that goes out the window in the case of a war. US law allows the freezing/seizing of assets, and other countries would have no problem with the idea.
So a situation could arise where the US simply declares China's holdings to be invalid and null. So long as the other bond holders are ok with this, and the (US) courts see it as legal, then China suddenly loses over a trillion dollars in investments. They can't just run off and sell them or something, they have nothing to sell. This would tank the renminbi and really screw China over. It actually could have a positive impact on the US, particularly if the other bond holders saw this as a positive (because the US owes less) and trusted that it wouldn't happen to them.
A country selling treasury notes isn't like taking out a loan with a loan shark. It works really different. US securities are:
1) Denominated in US dollars, and thus only worth something if the dollar is.
2) Payable on defined schedules, with no ability to "call in" the loan early.
3) Nothing more than promises to pay from the US government, and thus only valid if the government decides they will pay.