I don't think you can assume they are programming native code just because they are releasing support for iOS first.
Just because you are using React Native it does not necessarily mean you are supporting both platforms every time you create an app. You can, but you still need to work out the views for each platform. And this will still take a considerable amount of time to create, configure and test. Same for Xamarin but at least here, the UI can look more native to the platform, and quite honestly you will have access to more native OS functions. Qt is also quite good if you are a C++ developer.
There are tons of ways of building cross-platform apps and any decent software architect knows about this. It's more about what suits you best. There is nothing in my view suggesting that this wasn't developed using a cross-platform technology. Could be the case, but can't just assume that because the iOS app comes first.
Also, even with these cross-platform technologies you can still have issues in one platform that do not occur in the other. But yes, they make things easier hence why more likely it has been developed that way. They're probably simply focusing on releasing to iOS first, test it live, and then release to Android which has a larger audience. Just my 2 cents.
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