As far as I understand that's the definition of fungibility, right? Every dollar is interchangeable and identical?

So there's no functional difference between deleting $1 and creating $1 except semantics, compared to moving $1, as long as the total value doesn't change.

The government just deleting money and printing money to pay for whatever it wants suggests that those things aren't equal, which would be the problem if it were true.

Yes, they both create and delete money. That doesn't mean that the two processes need to be equal or balanced.

Your purchases do, or someone is owed their portion of the transaction. That's not the case when the government is writing bonds or appropriating funding to programs. They can create money freely, regardless of the tax they collect. Taxes serve a different purpose.