I'm not 100% sure about the total number of users the app can have. Because if a user has one or both of the iOS apps, they already have a token, so won't count towards the 100k limit, that's how I understand it anyway. So the guys at tapbots are obviously hoping that the tight integration with the iOS clients will spur Mac users who own the iOS Apps to buy the full app when it's released. So if we're working with the idea that a token from the iPhone/iPad app will act as your token for the mac app, rather than needing a whole new one, you could potentially have a lot more than 100k users. Judging from a recent blog, the guys at tapbots see 100k as a pretty high ceiling for the app, and are just trying to future proof it by pulling the download.
Another thing I just remembered. 100k is the point up to which they don't need approval from twitter, after that point they need twitter's consent to issue more tokens, but I don't know what hoops twitter will ask the devs to jump through, should they ever reach that point.