Well, they answer this:
"Visual Studio Code is the most popular editor. Vim is the most popular command-line-based editor.... All the instructors of this class use Vim as their editor. Vim has a rich history; it originated from the Vi editor (1976), and it’s still being developed today. Vim has some really neat ideas behind it, and for this reason, lots of tools support a Vim emulation mode (for example, 1.4 million people have installed Vim emulation for VS code). Vim is probably worth learning even if you finally end up switching to some other text editor."
So, basically, it's popular, useful, and they know it. But, even before saying that they provide this caveat: "As programmers, we spend most of our time editing code, so it’s worth investing time mastering an editor that fits your needs." Then they devote a little of time explaining how to best learn any editor.
Also, in the QA they say "The three of us use vim as our primary editor but Emacs is also a good alternative and it’s worth trying both to see which works better for you.... An advantage of using Emacs is that extensions can be implemented in Lisp, a better scripting language than vimscript, Vim’s default scripting language."