> Rendering engines are okay. Its the Javascript execution engine that is not allowed.

No, they must use WebKit:

"Apps that browse the web must use the iOS WebKit framework and WebKit Javascript"

https://developer.apple.com/app-store/review/guidelines/

You can render HTML content with a custom rendering engine - for example ePub ebooks are essentially bundled HTML + CSS content - there are several HTML rendering engines that read ePub ebooks but don't use Webkit.

Also, you can build a UI using web languages and render to native controls, all without using WebKit if you'd like.

Right, rendering HTML is fine. As long as you don't download it from the internet on-demand, or so.

Well, since you can't use the native javascript engine with a separate rendering engine, there's not much difference though is there?

(if you could, everyone would just do that to get dynamic behavior using javascript in their apps as a scripting language)

Not all content created with web technologies and renderable with an HTML/CSS rendering engine require Javascript