I do not understand why we get Litecoins after the swap ? If i wanted to swap bitcoins, i should get bitcoins in the end. If i get litecoins, i will have to then exchange them for bitcoins at the exchange and it will involve tx and trading fee. This service is only useful if you want to convert your bitcoins to litecoins and no intention of converting back to bitcoins.
As you can see on their platform they are some kind of hybrid between an exchange and a crypto mixing service: "SwapMixer is a privacy focused instant cryptocurrency exchange. Unlike traditional exchanges, our reserves are used to complete transactions instantly and all data is destroyed at the end of a session."
Also they do mention that they'll add more altcoins after feedback is received from the users. However I'm curious if they plan to add two-way conversions, so we can exchange LTC to BTC for example.