First of all, it's not free at all you need to open a channel by rent a paid fee on their DEX.

Also, the problem of "Lightning Network" always has maximum money you can send and receive so doesn't really helpful like regular trading when you can trade with a big-money. Now with a limited the amount by Lightning Network did you think they like "Uniswap"? of course not right. However, everyone who wants to use "Lightning Network" needs to understand how they work like "Open A Chanel" for your transaction, since almost different at all with DEX like Uniswap, Etherdetla, or IDEX.