Nit (maybe minor, maybe not): The EPUB standard requires[1] that the `mimetype` file be the first file stored in the ZIP container, and also that it be stored without being compressed.
It doesn't look like the author's code is conforming to this part of the standard.
If that is the case, then their Kindle may be able to tolerate the deviation. But using this code with an e-reader that adheres closely to the EPUB spec may produce broken results.