He needs critical mass in order to make this sustainable in the long term - without having to resort to ads, sponsored reviews and all sorts of ways to keep the quality.
The choice for iOS for the initial platform seems obvious: it's much easier to monetize quickly. And it's easy to add Kindle, Android, HTML5 and others later on - at the cost of maintaining multiple versions, provided that there's enough interested paying subscribers to justify the effort.
I think that's exactly the right approach, and a real step forward.