Most browsers don’t have a rendering engine unique to them. Apple made that a policy for iPhones and its derivatives, but a browser is more than its rendering engine.

So do I, but that doesn’t negate my original point: for almost every category of App that comes on a stock iPhone, you can replace it with a 3rd party alternative.

The exceptions are: SMS, Settings, and the Phone app for the numbers tied to your SIMs (there’s still a billion VoIP options including extra lines from carriers like T-Mobile’s Digits) and probably a couple others I’m forgetting. I think the Clock app has a couple of privileges not available to 3rd parties actually.

You can use a 3rd party browser, but that browser will use the version of WebKit that comes with the operating system (except soon, in the EU, whenever 3rd party browsers with their own rendering engines ship there).