If he is Nakamoto, I wonder if he's using this computing power to test strategies for divesting of the "nine-figure fortune" of coins which is assumed to be controlled by Nakamoto.
If he wants to unload the coins, it's pretty simple: make a series of transactions that are time-locked to happen at particular intervals. Because other market participants know the coins won't be unloaded at once, but at regular intervals, there is no panic event. The selling pressure (and more importantly, the knowledge of the selling pressure) would affect the price, but not in a catastrophic event.
I also think people regularly underestimate the role of ideology in what happens to those coins. Not everyone is so motivated by a fortune that they'd risk their wonderful creation. The fact that they haven't moved yet is compelling evidence, though obviously precludes nothing.
Satoshi has a large position relative to the whole market. If he every wanted to convert those coins to US dollars or some other currency, I'm guessing the supply it would introduce would cause the conversion rate to crash unless done in a sophisticated way.
Even a single satoshi leaving one of his known wallets could spook enough people to cause a crash. I can't speak for other investors, but even though I have been long in bitcoin for years, I would be out in a heartbeat, for at least a month.