They didn't actually make a mistake; they had no other choice. They couldn't just dump their PROM on a centralized exchange, could they? They used PROM with a nominal value of $5 million, to flood a Uniswap V2 pool with almost no ETH, where the pool had barely any liquidity left, and only got 0.0316 ETH in return, less than $100. This dazzling maneuver drove the price straight to the bottom of the pool, which actually benefited arbitrage bots: they arbitrage the PROM, which had been driven down to a low price, from the empty pool, and then sell it through other pools with relatively deeper liquidity.