If you read the article, you'll see their explanation: Vim is the most popular command-line editor (from a Stack Overflow survey) and all of the instructors know how to use it. The course is pretty *nix based so it makes sense to pick a command line editor and focus on it.
Pragmatically, Emacs is usually not installed by default on Linux systems and Nano sometimes isn't. If you had to pick a command-line editor to teach, Vi/Vim is probably the safest bet.