The problem is that the "we" who owns these unpaid Chinese treasuries are American citizens, not the American government.
They would, correctly, that those are funds owed by the Republic of China which is currently in power on Taiwan.
In my previous bank regulator career many years ago, I knew a banker/lawyer who, over the objections of the State Department, sued the Chicoms over defaulting on railroad bonds issued prior to the Commie takeover in China. To my incomplete knowledge, nothing was ever collected . . . but an interesting situation. He is long-since dead.
Sadly telling and accurate statement of the current state of our nation. The above shouldn't be possible in a nation governed 'by the people, for the people' is we are supposed to be. Our 'representative government' should tell China that we owe them NOTHING unless they pay what they owe US citizens.