It is only semi-decentralized, unfortunately. 100% decentralized DEXes are still on their way. Bitshares may be the only one right now. But even Bitshares has multiple ways to access it (through the app, or a web UI) and those are points that could potentially be hacked.
However, they can only hack the 'portals' to the smart contract. The smart contracts themselves keep the funds safe.. you just have to do your best to avoid compromised UI's or access points.
It is only semi-decentralized, unfortunately. 100% decentralized DEXes are still on their way. Bitshares may be the only one right now. But even Bitshares has multiple ways to access it (through the app, or a web UI) and those are points that could potentially be hacked.
However, they can only hack the 'portals' to the smart contract. The smart contracts themselves keep the funds safe.. you just have to do your best to avoid compromised UI's or access points.