Does Reg CC Apply to US Treasury?

The US Treasury is not a bank as defined by the regulation for Subpart C of the regulation, only Subpart D in limited circumstances:

For purposes of subpart C and D of this part and, in connection therewith, this subpart A, the term bank also includes any person engaged in the business of banking, as well as a Federal Reserve Bank, a Federal Home Loan Bank, and a state or unit of general local government to the extent that the state or unit of general local government acts as a paying bank.

Note: For purposes of subpart D of this part and, in connection therewith, this subpart A, bank also includes the Treasury of the United States or the United States Postal Service to the extent that the Treasury or the Postal Service acts as a paying bank.