How can they assume that the price per unit (aka 1 NXT) will stay the same if someone is buying up a large quantity of them? The more the attacker buys, the more the price will increase (exponentially) because other people will see the price rising and want to buy too. No one is going to sell their coins for "0" just because someone on the internet claims they can buy out all of PoS instantly - which they cant anyways, because not all coins are for sell on the market at any given time. For comparison, I think only ~1-2% of Bitcoins are moved a day (volume): http://bitcoincharts.com/markets/. If someone bought up that 1%, Bitcoin price would be in the thousands, with more and more people willing to buy and sell higher and higher.

Also another scenario: If everyone knows this guy has the money to buy out a significant portion of the coins, wouldn't they just hold because they know he wants a certain amount and will pay whatever price to get it? Basic market forces here, if everyone knows there is huge demand, they are going to put their selling price higher and higher until the supply matches the demand.

Last bit: Is the author's use of "cost nothing" an exaggeration? Like I mentioned earlier, no one will believe someone who claims he can buy out any PoS currency until he actually does it. Therefore, it does cost something, and the attacker would have to be willing to part with $millions-$billions of his own money. And then there's the fact that NXT is 90% resistant, so he'd pretty much be giving all (90%) of the current stakeholders a incredibly large sum of money.