I wasn't aware of that. But, due to the design of the network, you can't do naked shorting I would think. So, shorting would mean, borrowing actual shares from someones account and then selling them and promising to buy them back at a later date. I would also think that you would have to cover the sell in cash and would be subject to margin calls as well. I don't know if shorting BTC nor derivatives, could really ever "work" at quite the same was as they do in the stock market. For one, most people don't keep their coins in the exchanges - right there you can't borrow them for sale.
It works on Bitfinex because there's a pool of USD and BTC that's lent on a P2P system in exchange for interest. USD interest is very high and BTC interest is very low.