What do you mean by skin on top of WebKit? WebKit itself just renders the web page. The browser manufacturer has to build an entire browser and decide how to treat user privacy for example, does it have telemetry and what kind, whether to use native controls or not, what browser features to implement like reader mode, how to handle multiple tabs, gestures etc.. Calling that a skin is same as calling Vivaldi a skin of Chromium disregarding the effort made to make it different.
Well... Yes, Vivaldi is a skin of Chromium, that doesn't mean there isn't a lot of effort going into that skin.
The point is that Firefox and Chrome on iOS are not the same browser as Firefox and Chrome on any other platform.
For example, if I get a bug report from someone saying they're using Chrome on mobile, if they then say they're using iOS, that's a whole different kettle of fish from any other Chrome bug report I might get. It's going to be much closer to a Safari-related bug than a Chrome one, in a conventional sense.
For it to be a "skin", there would need to be a default "skin" to "reskin". WebKit has no default 'skin' becase it is just a web view. If you make an app with WebKit you have zero browser features, not even an address bar to type an URL into.
Skin, UI, I take your point, but the point being made elsewhere is not about the semantics of "skin", but the fact that the underlying browser runtime and renderer is not the same between iOS and other versions of browsers. You're unlikely to find a bug report that is caused by the UI of a browser, rather than the runtime.
