The United States dollar is not owned by the US. It is a private currency owned by the US Federal reserve which is a private institution.
USDT is a private currency owed by the tether corporation
If you live in the United States, you are paid in a private currency every day in the form of federal reserve US dollars.
You can use a private currency to pay your taxes and you can take out loans that are payable in that currency from all US banks.
Tether directly competes with the federal reserve dollar, which is a product of the US federal reserve.
US treasury bills back tether in addition to other assets.
Nothing backs the US dollar except the legal history of being able to pay your debts with it, and the usefulness of its function within the banking infrastructure.
Tether is directly aligned with the United States treasury department. The federal reserve is in direct opposition to the United States treasury department.