I thought that no other browser engine could be provided on iOS. so no ladybird's engine, no servo, no gecko, no blink, only webkit
that was true until a few months ago due to a ruling in the EU. It's still currently the case that only WebKit can run on iOS but they're gearing up to change that
Some geographic regions have declared that not allowing other OS engines on iOS is anticompetitive so they're requiring Apple to allow them.
Apple is fighting it tooth and nail and coming up with requirements for other engines is a small way of doing that.