You can only pay yours debts with the government with money created by the government. And this is not something new.

Government money is, also, a liability in a very real accounting sense too.

When the government creates money (by crediting the accounts of their providers, for instance) it is also registered as a liability for itself.

This same money always will be accepted back as a payment. That's the promise.

The promise is to accept for settlement of tax obligations.