It seems to be better than any exchange out there, on paper i guess there will be a way to demonstrate the funds are locked (peraphs is in the whitepaper, i haven't look yet). If everything is working, is going to be very hard for an attacker to obtain both privte keys.

Is it true they can deny you access to your funds but on the other hand they cannot either access them, consider when you are sending now funds to an exchange you need to trust the exchange is secure and they will not run away with it as well, so basically trust 100% the exchange.