You're not contradicting me. The section you're quoting is not about this, by the way. You want section 2.5.2:

"Apps should be self-contained in their bundles, and may not read or write data outside the designated container area, nor may they download, install, or execute code which introduces or changes features or functionality of the app, including other apps."

Yeah, but if you read carefully, section 4.7 explicitly says that you must use WebKit and JSC to run third party software. Therefore, you can use your own engine to render your app (see games or Google's Flutter), but you cannot use it to render third-party content (thus making impossible to create a browser)

Oh interesting.