It seems to me that you haven't written an application of similar complexity. Also, Ulysses is not one app, it's a Mac app and two mobile apps, for iPhone and iPad, yes they are quite different. Have you actually used the app? Seen the customizable theming system? The export to PDF/Text/ePub/docx/HTML? The way you can rearrange and group sheets? The smart folder system? And most of that is present in the mobile apps, too.

You are underestimating the complexity of writing, maintaining, documenting, supporting and marketing apps of this size. I think my guess of ~5 people is about right, they might be able to do it with a smaller team, but probably not less than 3. I'd say 2 developers and one person to do marketing/support is a minimum.