Were you using exclusively free plans to build your app?
I didn't find app building to take that long, even without knowing the ins and outs of kotlin and swift.
I did find the process of publishing the app to be one of the hardest things. The last 10 - 20 percent of app development still requires a lot of paperwork of you aren't already a developer.
If you read the article, you'll see that the author took the same approach one would use when writing an app manually from scratch:
> I started with a simple hello world application. Then added in the habit list, the ability to add a habit, edit a habit and so on.
This hands-on, incremental approach works, of course. But I don't think it is the most effective way of using LLMs to write apps.