This would be like saying that since bread, milk and fish are freely tradable in an imaginary barter economy, there is no need for money in that economy to solve the DCoW problem.

Yes, these crypto-currencies are tradable, however, the high-barrier to liquidity makes it significantly harder to get new currencies to the point of high-liquidity, where the price-discovery process becomes reliable.

Bancor protocol solves that very problem, thanks to the new technologies which enable anyone to create programmable tokens (smart-contracts).

The very definition of DCoW is that two parties with opposing wants are required in order to complete a transaction. Exchanges merely deal with this problem, but by no mean they are solving it like money did for barter.