If someone is trading between USDT and bitcoin they are not going to be doing it on the bitcoin network. They are going to be doing it on a CEX where there is no confirmation time or they will be using a derivative such as wrapped bitcoin on another network such as Ethereum.

The 30 minute wait would only make sense if you were to trade USDT for bitcoin on a CEX, then send that money using bitcoin to another CEX to execute the next trade to USDC.