Not necessarily true ... depends on the net position that Plus500 hold, how much risk they take on their book and how much they hedge on Gox or other exchanges if they are net short. Not sure how they deal with net long unless they are able to borrow BTC to short.
I think their business model has error :-). They do not buy or sell bitcoins. You cannot buy/sell bitcoin at plus500 ... only make CFD.
edit: you can after CFD manipulate price easy (if you have enough resources)
YOU cannot buy or sell Bitcoins with them .. it is all CFDs. That does not mean that THEY do not use BTC to hedge when and where they can, and I would be very surprised if they do not
I'm not sure they can DO HEDGE (they do not have enough bitcoins) ... beacause they do not know how much I'll short at their platform (and they need 4x more ... if I'll try short 12M btc then they will need 48M. ... and there are only 12M existing ).