Sadly, you've been sold a lie. By the media telling you to expect cross platform everything... a media controlled (and effectively) owned by Google, a company that stands to benefit more from creating this expectation than anyone else. Hence their reasoning for attempting to create this expectation.
There is NOTHING easy about porting anything to any other platform.
- concede performance, from the beginning, and size (everything will be huge and unwieldy) in order to have chance* of being cross platform... but you still build for one platform 1st. Always. Or go insane.
- aim for size, efficiency and performance and forget about cross platform until you can afford to completely rewrite for any other platforms.
That asterix*... there's no sure thing... even if you make these sacrifices, when it comes to getting things to work cross platform.
Please know this: hyperPad took option 2, very wisely.
Objective-C is really STILL the only language capable of running something as elegant as reloading an operating environment within an environment in iOS, as hyperPad does. If you were to try this in Swift you'd be chasing your tail, all the time. Swift isn't nearly finished, either. They still break things with each significant upgrade.
Objective-C is not cross platform. Not even close to it. Beautiful language, far more so than C++, and make C# look like the utterly verbose remake of Java that it is.
There aren't other game engines (other than what they're using, which is cocos2D-iPhone) able to do what they're doing... except SpriteKit and SceneKit, both of which are expressly Apple, too. So no cross platform when choosing that path, either.
Cocos2D-X is a layered, contrived myriad of byzantine dependencies and general build and administration disasters I would not wish upon my worst enemy. And that's if you're only targeting one platform with it. Aim at 2 and you'll need to hire someone just to administer the setup and maintenance of compiling and using the thing. It's a MESS, because it's maintained in a country where teams are big and money is thrown at programming projects at a size and scale that would make you blush, and where workers work HARD.
Porting to Mac engages almost zero people, and makes for no benefit... it's a touch screen focused coding and creation environment, in which there's nothing fun about porting to a mouse driven world. Exporting to Mac... possible, but still requires that they own several Macs, from each genre, with multiple versions of OSX installed... test, test, trial... test... etc. And they can't ship updates now... so.... nah.... probably not.
Porting the exporting to the Apple TV is the easiest thing as tvOS is fundamentally very similar to iOS... but... wasn't a thing when cocos2D-iphone was built and last maintained... and it would still involve an enormous amount of testing and then there's problem solving with the input devices of Apple TV... which, thanks to Tim Cook, are ridiculous. Bad. In the extreme. And handled worse. It seems nobody cared about this, because he didn't.
It's not reasonable to expect a porting from iPad to ANYTHING else anytime soon. Enjoy that it's on the iPad and continue to beg them for Undo in the Behaviour Editor and a fix of the "game loop" so your battery lasts longer and you get a stable 60fps everywhere.