Basically they use leverage in CME for their bettings, but when buying real bitcoin, they will buy in exchanges like Bitstamp and hence they can only do what a whale would do.
Why would they need to buy real bitcoin, when contracts will be settled in fiat?
Because if they want to short the market they can artificially inflate the price by buying shitloads of it and driving the price up. Once the price is high they can short it, dump what they bought and then do it all over again. There's some graphic floating around that says this. Is it true? Who knows. Does it seem like it could happen? Perhaps.