I don't think this would be the case, since these firms are actively market-making using USDT. I.e., they're not just turning fiat into crypto using USDT, but t

I don't think this would be the case, since these firms are actively market-making using USDT. I.e., they're not just turning fiat into crypto using USDT, but they are also market-making in USDT/BTC, USDT/ETH, etc. My understanding is that Alameda is trading with fairly high frequency, so they would keep funds in USDT in order to facilitate those trades. Further, the on-chain data suggests that they don't trade out of USDT to USD – otherwise we'd see high corresponding USDT burns.