One person can't do it. When you're making FOSS you're standing on the shoulders of giants. Most of the code that makes up this app wasn't coded by the developer of this app. Also, there are multiple people committing code to this repository, so it literally isn't just one guy developing this app specifically.
woops sorry quite honestly I did see the iOS logo but because its on github its my first time seeing a project thats iOS only, with no android alternative thats also open source. Thought they also had a branch for android.