According to the IRS page, it seems like the answer is a no, as cash is neither US real estate, tangible property or securities of US companies.
U.S.-situated assets include American real estate, tangible personal property, and securities of U.S. companies. A nonresident’s stock holdings in American companies are subject to estate taxation even though the nonresident held the certificates abroad or registered the certificates in the name of a nominee.
Exceptions: Assets that are exempt from U.S. estate tax include securities that generate portfolio interest, bank accounts not used in connection with a trade or business in the U.S., and insurance proceeds.
However, the Boglehead wiki suggests otherwise
Interactive Brokers is a popular choice for many, but be aware that it is US based. This means that you should never hold more than $60,000 in cash at this broker, otherwise US estate taxes again become an issue.
Based on this documemt from JP Morgan
Non-bank deposits, such as cash accounts in U.S. brokerage firms, are likely to be subject to U.S. estate tax.
For simplicity and generality, assume that the person has established permanent residence in a country with no US estate tax treaties.
