I am guessing one “Killer App” is: The Rule of Law.
From the transcript of the presentation: "I want to explain to you that there were six killer apps that set the West apart from the rest. And they're kind of like the apps on your phone, in the sense that they look quite simple. They're just icons; you click on them. But behind the icon, there's complex code. It's the same with institutions. There are six which I think explain the Great Divergence. One, competition. Two, the scientific revolution. Three, property rights. Four, modern medicine. Five, the consumer society. And six, the work ethic."
Since property rights are ensured by the Rule of Law, I'd say your guess is on the mark.
He specifically stated property rights. Otherwise, people are reluctant to invest or unable to do so.
Scientific revolution
Property rights
Modern medicine
consumer society
Protestant work ethic
Competition/capitalism