As far as I know Apple doesn't require browsers to use the same rendering engine as Safari. It's just that so far any rendering engine + JS engine required a JIT to make it performant. And because a JIT is not allowed because apps are not allowed to write to executable memory. That's why the 3rd party browsers all used the same rendering engine as Safari.
The app store rules disallow other rendering engines: 2.5.6 Apps that browse the web must use the appropriate WebKit framework and WebKit Javascript
Before WKWebView, even the Safari-based UIWebView didn't JIT JavaScript. Chrome for iOS was released years before WKWebView was added, so a lack of JIT in their own JavaScript engine would not have made a difference.
