If Tether goes completely bust, it's likely that closely associated exchanges won't be able to cover the bets.
Until that happens, they control all the levers and can just move the market as necessary to make you lose your bet.
Also don't understand what moving the market means in this context. The market is at 1:1 for USDT/USD, if there's selling pressure that would lead USDT to trade under $1, the only way to manipulate that is to buy lots of USDT from everyone who wants to sell, which tends to support the peg. Not even sure why that's called manipulation. If there's a real panic and tether turns off withdrawals there's no way anyone has the money to buy everything up at peg.