That doesn't really seem to follow, to me. The worst a single creditor could do is not bid, or bid for higher rates in future treasury auctions. The terms of existing debt are fixed. The impact of this wouldn't be large unless other creditors followed suit - there are many parties interested in buying up US debt.
Furthermore, if a holder of US debt declared war on the US, I wonder if that wouldn't be viewed as a credible reason to default on that outstanding debt - certainly doesn't make much sense to be sending interest coupon payments to someone invading your country.