Yep. technically, Apple doesn't force Firefox to use WebKit. They could still use their Gecko WebRenderer and use Apple's JavaScriptCore to run page scripts. Same goes to Chrome: Blink + JSC instead of Blink + V8.
In practice in seems like both Chrome and Firefox have their renderers and JavaScript runtimes so tightly integrated that it's very hard to replace the later.
Elsewhere in this debate sharpneli says that there is no public API to call into JavaScriptCore and using non-public APIs violates the App Store rules. So that would explain it.
In practice in seems like both Chrome and Firefox have their renderers and JavaScript runtimes so tightly integrated that it's very hard to replace the later.