IDEX is semi-decentralized: order-matching is handled off-chain, funds must be deposited into a centralized smart contract that handles all settlement, but the smart-contract itself and balances within it are held on the Ethereum blockchain. It prevents some forms of exit-scam or hack (you don't need to trust that the IDEX principals won't send your funds to a bank in the Bahamas), but retains vulnerability to a bug in the IDEX smart contract allowing a hacker to obtain them. The IDEX smart contract is at least public and auditable, though.
There are 4 core components to a decentralized exchange:
(1) storage of funds, (2) order books, (3) order matching, (4) settlement. Unless you have all 4 of these it is a hybrid exchange. Calling it anything else is misleading an disingenuous.