Here is the argument: A higher difficulty makes a 51% attack on the network harder to achieve. Since that is widely recognized, a higher difficulty makes people more confident in the long term viability of bitcoin, which leads to more hording and acceptance, and THAT drives up the price.
That makes no sense. You could equally well argue that ASICs coming out makes it easier to attack Bitcoin because the cost to achieve a 51% attack goes down. This is perfectly cancelled out by people who buy the ASICs raising the difficulty. The net effect is that the difficulty to attack the network is driven by the price, not the other way around.
An attacker could be buying ASICs for all we know.