I'm willing to bet that there is a huge overlap for iPhone and Twitter users, and for that subset, Tweetie can most definitely be considered a killer app. I get what you're trying to say with them mentioning a killer app be geared towards the user of a specific site, but really, that could be said about any killer app. The fart app is geared towards iPhone users AND fart sound lovers. iMob is geared towards iPhone users AND mob-style game lovers. All these apps have done wonderfully, and are apps I would consider killer. The point is, an iPhone app is killer because it appeals to a subset of users really really well.

Additionally, I see nothing wrong in them pointing out Tweetie, even though it is one of billions. Design and user interface is a very important layer to take into account, especially with these iPhone apps, and Tweetie's design is very intuitive, in my opinion. Design has much to do with an app being killer as its underlying technology.