Procoding uses Processing.js that runs in the built-in Safari Mobile browser. Because of any browser’s security restrictions, reading and in particular writing local files is practically impossible (you really don’t want some shady websites you visit accidentally saving files on your hard drive).
I don’t know the exact implementation of processing.js (would have to go to github and look up the code for saveStrings), but I suppose they are using localStorage, something like a cookie in the browser’s database.
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