Don't forget that crypto exchanges have none of the properties of cryptocurrencies themselves, in terms of censorship resistance and cryptographic guarantees. Since fiat currencies are involved and these are businesses operating in countries with laws, they have to enforce whatever the government wants to require of them. They can absolutely halt trading, or freeze funds or assets on their platform at any time.
DEXes, however, can be designed so that no person can halt trading or freeze your funds (usually you trade out of your own wallet anyway). A DEX is just a computer program running on the Ethereum blockchain. Once deployed, the blockchain is immutable, and it just sits there accepting orders.
