Were they stolen and still could be in circulation?
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.
If the coins were stolen, I believe it would have to have been done in a situation like you just described. What I can't understand is the stupidity involved. In order for this to happen all the way until you have only a couple thousand BTC left to your company's name, you have to be completely and utterly incompetent not for weeks or months, but for years on end. You would have to never ever realize that "hey I am bleeding money and not making any." They have to be almost impossibly stupid for this to happen. I don't know if we will ever get the full picture.
