I think the Appstore was planned all along, just did not fit in the first release, so they adapted the launching narrative to: "the browser is enough for all 3rd party software".
Once native APIs were available, that was true, but before it was even clear that the iPhone would have an app store, they very much did let it flourish.
[0]: https://www.macrumors.com/2021/03/03/scott-forstall-pandora-...
[1]: https://mjtsai.com/blog/2026/04/06/apple-creating-all-the-ap...