My own definition of decentralized exchange is that any coin be present in the exchange and traders does not need to comply with KYC requirement, so we can trade anonymous unlike in centralized exchange where we are like bank clients. Unfortunately, things has change, with the government regulation, slowly the anonymity idea is not that attractive anymore, so some exchange even if DEX as introduce but not really a DEX in reality, good example is Binance DEX where some countries based on IP address are not allowed to access the site.