This is a good point - the issuer of Tether (iFinex?) could burn USDT that they hold without paying out to currency. I'm not sure how holding Tether would benefit the issuer of Tether, though, since anytime anyone transferred USDT to the issuer they would expect a cash payout. So I guess my statement "USDT is backed 1:1" should actually be "circulating USDT is supposedly backed 1:1 by USD." Any "burnable Tether" should be subtracted from the Market Cap to find the actual "backed Tether."

I don't know of any way to find how much USDT is held by the issuer of Tether (or their coparties) and how much is held by other wallets at any given time. I suspect that calculation would require a significantly greater knowledge of the blockchains that USDT is on than what I have.