From Mathematical point of view, in both cases, market caps are the same but the distribution of wealth are not.
Your argument is only true when burning occurs in every CET accounts where number of burned coins is proportional to number of owned coins.
The situation is different when Coinex buy the coins and burn them (not burn yours).
I can give you an example. Let's say total market cap for CET is 100USD, total coins is 100CET, you and coinex share the same amount (50 CET). Suppose coinex buy coins with 10 USD, new market cap is 110 USD.
Case 1 : dividend and redistribute
They redistribute the bought coins to you, so both you and coinex have 50 CET. You have 55 USD = 110 / 2 USD.
Case 2 : burning the bought coins
They destroy some coins so you own 50 CET, but coinex owns ~40 CET. Total market values is still 110 USD. Now you have 110 * 5/9 = 61 USD. more than 55 USD.
That is the reason why people prefer burning to dividend.