Allowing a custom web browser is different than allowing a system wide custom WebView... Since you can't currently do anything similar that's a weird complaint.

For example, I have Google Maps installed, but if I make an app and add an MKMapView it is the same old Apple Map it is everywhere.

It seems unlikely that a non-browser app would embed some other engine. I don't think I've ever seen that done on the Mac, for example.

What about every Electron app?

Does it count if it's not being used to actually browse the web? I believe Apple would allow that even on iOS.

In the case of Electron, the application is the webpage. Apple allows web apps built with Cordova on iOS, but warns that they may be rejected in the approval process if they run too slow.

I kind of doubt it. My understanding of the rule is Apple will not allow any alternate web renderer on their platform. You might be able to get away with the old opera mobile thing of having another server render the image and just serve the image on the phone, but no one wants to actually do that.

I'm puzzled at how this reply coexists with the other, older reply next to it pointing out an example of where Apple does allow exactly that.