You are correct that this was my assumption and I have made this assumption because of how easy it is to program in swift. The shoutback app was made as a University programming competition which leads me to my assumption that the app was only developed for iOS distribution. A small dev team (one or two people I believe) is more likely to be focused on a singular platform.
Developing views in React is still very fast in comparison to native code. Yes there are several cross platform development libraries/technologies and I have used a few. None of them are as good as React in my opinion.
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