That's not a definition for real. Requiring to pay taxes by US dollar makes it more in demand. So it creates some demand for the dollar. It is the same way by which bitcoin or any other asset gain value.
That's why I put it in quotes. However, calling it "some demand" severly understates the significance of this fact. It's not just some demand, it means it's absolutely vital for vast majority of people and businesses in US. Which won't be the case for bc unless it becomes legal tender