This is a very interesting project. I visited the site, but there should be an article about, how this works. Atomic swaps do not apply to all blockchains, and I think that the Placeholder cryptocurrency, is used for this reason. If the blockchains support atomic swaps, like Bitcoin, Litecoin and Decred, where atomic swaps have been tested, the transaction can be done without an exchange. If there are different blockchains, I think that you buy Placeholder cryptocurrency with an atomic swap, and then you exchange Placeholder cryptocurrency with an atomic swap to buy the desired cryptocurrency.