No, U.S. Treasuries are not counted toward the US$60,000 U.S. estate tax exemption that applies to the estates of non-U.S. persons. Cash (in any currency) held at a U.S. broker is counted toward that exemption.
For example, let’s suppose you want to invest in U.S. Treasuries through a U.S. broker, and that you have US$100,000 you want to invest. Let’s assume you have no other U.S. assets that could be subject to the U.S. estate tax. (No shares of Apple stock, for example.) You may wish to deposit US$50,000 (below the US$60,000 exemption), buy a U.S. Treasury, deposit the remaining US$50,000, buy another U.S. Treasury (with a different maturity date), and loop/repeat such that you’re never above US$60,000 in cash....
....Or you can just buy more life insurance if you’re concerned about the U.S. estate tax liability.