I rarely see a situation that a project lies that they are decentralized while they are centralized.
There are many of them. Many of the bitcoin P2P exchanges are not actually decentralized but just very few of them are. You may not know because you may not be able to differentiate between centralized and decentralized exchanges accurately.
It is near impossible for an ordinary user who sees only through the screen to differentiate between a centralized and a decentralized exchange. A novice can only spot the difference with keen observation, especially about the user friendliness of the platform. My curiosity is that, if a centralized exchange claims to be decentralized, at least there should be developers to test and refute their claims.
As far as I understand, cexes are where exchange holds user funds and dexes are where you are in custody of your funds.
Cex would have login system like username/email + password and cex server authenticates and gives you access while on dexes you can login using your wallet and some may have their own wallet system where you are given wallet keys, so even if frontend were to be down you would not lose access to your funds.
Also, dexes run on blockchain where everything you do is recorded on blockchain — at least this is case with EVM dexes, not too sure about platforms like Bisq.
So, it's rather easy to distinguish between two as in the end it boils down to whether you are in custody of your funds or not.