If the background to that rumour is true and its been actioned over a long period, then it's likely the perp sold the coins as they aquired them. That is, they're not sat there holding 750k BTC but rather the price over the last few years has been depressed by those being sold. The theft then only becomes obvious now and those who thought they had coins on MtGox didn't really hold anything (flashes of fiat reality in my head), coupled with that all holders of BTC who've been spending in the real world on product or back out to fiat, have lost money to those theiving.

Which then is a good sign perhaps that the appitite for Bitcoins was sufficient to absorb that amount and still show a healthy rise. We'll have to wait and see now, if then the price of coins avaliable without MtGox interference, suggests there really is more demand than easy forged supply.