You don't HAVE to edit directly on the server with Coda, but it is an OPTION. You can just as easily work locally/commit and then sync with the dev server.
Also, not everyone works on projects hosted on a vps. ftp/sftp is very much still relevant in the real world.
Diet Coda is not meant to be a replacement for your macbook, that is why it is 10 bucks. Also recreating a virtual local file system with git inside an ios app is probably not worth the effort. I for one welcome the "quick hack" nature of the app. Its not meant to be an ide replacement.
"Much as I love Panic, I’ve never used Coda" then why are you writing a critique on software you have never used?