Decentralized ledger is the buzzword to say blockchain in this instance it's the exchange's bitcoin wallet, the exchange in itself is centralized.
I quote their website: "Fully Decentralized Exchanges: An Unrealistic Solution
Transactions between different blockchains and fiat transactions are impossible on decentralized exchanges. This is a significant drawback to their use given that transactions within the same blockchain represent only 3% of all daily transactions.
Although some projects have been working on the cross-chain issue, a satisfactory solution has yet to emerge, and in any case their potential future expansion is limited to 25% market share."
Their whitepaper's title is "Legolas Exchange A DEMONSTRABLY FAIR, PREMIUM CENTRALIZED EXCHANGE USING DECENTRALIZED BLOCKCHAIN TECHNOLOGY"
https://legolas.exchange/static/public/legolas-whitepaper.pdf?v=11