It's not just the taxation (which is true), it's the fact that they are backed by a particular entity. In the case of USD, it's the United States government. Dollars may not be "backed" by anything tangible, but they are accepted because the US is (largely) a trusted entity. That's what makes USD useful to people internationally.
Bitcoin doesn't have the advantages of having a government backing it, but some would probably argue that that is an advantage.
Considering how many trillion definitions people may mean when they say a currency is "backed by" something, I don't think your explanation is an improvement.
For that reason, I recommend listing the specific things that make the USD valued: in this case, the need to pay USD in taxes, and a very wealthy entity (US government) prefers to trade in that currency, or whatever specific thing you mean with "backed by" in that statement.
Bitcoin doesn't have the advantages of having a government backing it, but some would probably argue that that is an advantage.