There's some nuance there though, "killer app" is sometimes short for "application" in the sense of "use" not "program"[1]. When Steve Jobs said "the killer app is making calls." in the iPhone intro, he wasn't talking about the dialer app.
Later on the App Store could certainly be credited with Apple's success, but not during the iPhone intro, though? I think people had been saying "our killer app is X" (where X was not a piece of software but an approach to doing something) for quite a while before that.