I hoped that it would be possible to have LT create a PDF from an uploaded IDML too. It doesn't (yet). I've contacted the supporting company behind LT.
Just had a friendly conversation with Zsolt from Kilgray, to which I replied:
Sorry, you're absolutely right: both the built-in Unarchive tool of macOS Sierra and the popular Mac unarchiver The Unarchiver only extract an IDML file. However, when I open the ZIP archive in Windows 8, I can also see the PDF. (Perhaps there's something special in the structure of the ZIP file?).