Simple: JIT compilers are banned and so that excludes any modern browser's JavaScript implementation from iOS. But anyone using Apple's JavaScriptCore has nothing to fear.
The rules explicitly forbid any HTML renderer or JS interpreter aside from WebKit, JIT or no JIT. I believe all the popular third party browsers today still use the non-JIT UIWebView rather than WKWebView because the former gives you more control over the request cycle
WKWebView has JIT. In executes the JS in a different process than the hosted app; there the JIT lives, this special process is whitelisted to do JIT magic.
I understand that, the original OP was stating that none of the "browser skins" had JIT because they all used UIWebView, which isn't the case with the link I posted :p.