I also think he means DEX. In a few months or years there will be better ones where you can trade coins not only ethereum tokens.
You are right, IDEX exchange every day becomes more popular, but in order for their servers to withstand the load they need to think how to stop soft-bots from trading.
IDEX is a hybrid exchange. The final settlement takes place on IDEX' systems (explaining why IDEX was unavailable a while ago), introducing a bit of a black box. So I am unsure IDEX would qualify as sufficiently transparent for Islamic banking.
That only leaves the fully on-chain decentralized exchanges, and perhaps over-the-counter (over-the-internet)?
Probably yes, but OTC is a pain in the ass and a reason why exchanges exist in the first place. A Dex supporting atomic swaps would be the best solution I think. Something like the Komodo exchange.