thanks for pointing out that service. never heard of it before but i think the problem with services like this (DEXs) is they typically have low volume which means if you want to buy any significant amount of btc ( or sell), you're going to get price slippage.
This much is true, but if I was selling a large amount I would never go to the order book on a single platform and just start scooping up all the orders there. Much better to create your own order, preferably two or more orders on different platforms, setting your own price and letting trades come to you. Then you can fill your order with multiple different parties with no price slippage.