I think they are using existing DEX because existing DEX don't have the ability to TRADE BTC with ERC20 tokens . Also existing DEX are well established and have liquidity to make it very useful right now.
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This is correct! Although we are working on our own inter-blockchain exchange (
simply called LQDEX), LQDEX Bridge is built with the intention of assisting existing DEX. Currently, it is not possible for users on DEX, like EtherDelta or Airswap, to easily trade direct pairs. LQDEX Bridge is an auxiliary for users on these exchanges to have the ability to trade BTC with ERC20.