As briefly referenced in the article SushiSwap was able to steal roughly $1B liquidity from the Uniswap platform by taking their users and offering various incentives for them to move their funds
It wasn't a 'steal'... it was an incentive to move, which was only the cost of a transaction. Liquidity providers are looking for the best rates and most amount of transaction fees, which far outweighed the cost of the transaction to move. The bar was very low. It was also brief and only lasted until the next thing came along...