Exchanges are a textbook case of a shared ledger. They have separate systems that must work in sync with very low latency between them: order processing, the ledger (for clearing) and price advertising.

The main advantage of a blockchain is with the ledger price advertising between exchange members comes free.

I guess they can even implement OTC markets between peers, but I am not sure how do they plan to work with dark pools where parties want anonimity. Maybe they will just implement an internal blockchain with APIs for anonymous trading.