Not that minor, there are indeed 2 different engines.

One for Safari and one for the rest.

You're parroting old news. The different engine is for UIWebView which is now deprecated in favour of WKWebView which uses Safari's Nitro engine.

Websites saved to the home screen use UIWebView's engine — but that's not the discussion, the discussion is about browser apps which can and do use WKWebView.

Right, but that comes out of the exact same code with some minor feature flags.