I guess the point is that theoretically PoW could be attacked, but it would be vastly less expensive to attack PoS. In fact, the greater the attacker's credibility, the cheaper the attack would be.
Slight issue. With POS (at least in an implementation such as NXT), by the time you bought up enough of the coin supply to "attack" the network catastrophically, the only holder affected significantly by such an attack would be yourself

Put another way, it would be like buying somebody's house off them so you could burgle it
