If you live on the same planet as me, reserve currency (the one that international trade is settled in) of this planet is still USD (although there's been steps by other countries in the last 2 years, especially China, to move away from USD), which is a fiat currency. We don't need to theorize here of what reserve currency can or cannot be. We're talking about what it is now. USD, which is a reserve currency of this planet, is fiat now, backed by nothing, unless you count aircraft carriers, of course.
You are missing the point again. The initial discussion was about why trades with BTC can be taxed. You are going off the topic. Go back and read the posts again, locate yourself. BTC is also "backed by nothing". This fact alone do not hinders in any way the capacity of elected governments to collect tax from trades done with BTC.