If it turns out that the "lost" 800 kBTC still exist and can be spent by someone, that will definitely push the prices down.
The lost 800 kBTC don't exist. Mt Gox doesn't have them. 800,000 Bitcoins that people thought were held at Mt Gox we now know did not actually exist. The only Bitcoins that do exist were already in circulation, already pushing the price down.
Is this a fact or just a wild speculation?
It's speculation. But I believe it's the most consistent explanation for the information available -- Gox was believed to have over 700,000 Bitcoins when in fact they didn't have anywhere near that many. This "deficit" accumulated over time.
Gox seems to be claiming that they lost the privkeys to their cold wallet. Good job, if that's the case. A sane company holding that kind of money would have created some kind of multiply-redundant offline solution and spent a lot of money for keeping the privkeys safe. A million dollars a year would not be exaggerating it.
Where do you see them claiming that?
The thing is, Gox's business model was actually valid and sound, and they made quite a bit of money in the past couple of years. Even though they lost some of it, unless the fiat and coins went somewhere, they ought to have had a lot more than the 2kBTC and the $20 million. The BTC holdings should be at least several dozen kBTC.
The problem with the Bitcoin exchange business model is that proper security is expensive and annoying. In the past, customers have believed exchanges when they simply say they are secure. So there was a perception that there was no need to spend more money on additional security -- it wouldn't bring you any more business. The problem has been that there has not been a significant financial incentive for exchanges to actually be secure.
If it turns out Gox actually lost access to Bitcoins that are now stuck in wallets nobody has the private keys to, that's a different situation entirely. Either way, it should be good for Bitcoin in the long term.