That analysis has a major flaw. One of the main advantages of a centralized exchange is that they don't have to process all transactions on chain. As such, this is measuring very different statistics for centralized and decentralized exchanges.

That is true. Probably by pure volume of trades the CEX order books will always be able to facilitate an order of magnitude more than DEX as they can fulfill orders instantly with almost no fees at all, and many beginners and casual traders with crypto will not bother with or even understand what it means to transact on-chain.

But there is a very significant amount of crypto trading occurring on-chain, to the tune of multiple billions of dollars per day.

It's hard to know for sure, but it wouldn't be surprising if the majority of on chain trading is between different accounts of the same person in a (bad) attempt to launder the money.