They specifically ditched react-native because of their requirements regarding file system traversal, so this is definitely not something that could have been done in-browser.
But unfortunately not on Safari respectively the iOS webview, which would have been mandatory for the author to use.
If I am wrong, I‘d gladly take a solution. I think this is one of the main problems for PWAs: No good , platform-independent way to access the local file system. As in „pick once, access forever“.
https://stackoverflow.com/questions/64283711
It works nicely in Chrome on the desktop and on Android. Not sure how the situation is on iOS.