The IOUs are being traded, right? So, they´re fungible at least if that means mutually interchangeable. Maybe I´m misunderstanding something.
they have no blockchain to transport the actual token from one exchange to the other, thats why they are not fungible.
And I also think, they could have serious problems paying out the tokens. Maybe they even sell more, than they have... you never know...
Then it means, that you have a token, that is not fungible to a real LISK later, if the prices rise too much and the exchange defaults...