We already have these problems in the normal stock exchange -- they're called "whales". In order for whales to capture the best value to _them_, they trade carefully and slowly so as not to shock the market.
The problem is all transaction on bitcoin are public. Satoshi even moving his coins to begin trading would cause a massive freakout. Coinbase even had to disclose this risk in their IPO filing.
You're right. That would be more akin to massive, instantaneous inflation which could shake peoples' confidence.
That would really depend on what Satoshi would do exactly with his/hers holding. It would probably cause volatility, but imo not larger volatility compared to what we are already used to.