My understanding is that third party engines are allowed. They just can’t JIT the JavaScript unless they use the builtin Safari one. So, if you want to have the same performance of the builtin Safari browser, you really need to be nothing more than a skin around it.

This is false too. "Nothing more than a skin" suggests you can't make a full-featured browser, with your own code, that you're just replacing the Safari UI. This is simply not true.

I literally said that they are allowed. They just can’t JIT the JavaScript, so performance suffers compared to Safari. So, if you don’t care about JS performance, then yes, you can make your own full browser with your own rendering code.