At any moment they can see the order book size in btc and the prices so if they buy btc with USDT they can safely hold the Bitcoin provided they are monitoring

At any moment they can see the order book size in btc and the prices so if they buy btc with USDT they can safely hold the Bitcoin provided they are monitoring the order book. If a big sell order comes in or someone tries to cancel a buy order they can preempt it with a sell from their holdings to make sure they don’t end up in the red. Bottom line if you can freely frontrun an order book you can make a lot of money. USDT provides the liquidity to do that.