The collateral the Federal government offers is the U.S. economy, which it can tax, and national assets, which it can lease or sell. From the perspective of international global finance, the U.S. Federal government is just another counterparty, albeit a very large one.
The fact that the government prints its own currency doesn't change things that much in the abstract. A dollar note is just that--a promissory note, a type of negotiable instrument like a corporate bond or personal check. The U.S. government can print more notes, but a corporation can print more bonds or a shopper sign a bunch of checks. The value isn't what's on the face, but what people will give you in exchange for it. In the case of the U.S., foreign parties are still eager, perhaps too eager, to exchange automobiles and semiconductors for U.S. notes. Ditto for U.S. bonds.