I'm surprised they don't even mention vimtutor. It's preinstalled on every machine with vim (to the best of my knowledge). This seems like a cool project, but might as well give a shout-out to the original concept.
vimtutor is to Babbel what this is to duolingo. Many will prefer learning through a game but some want a more textbook approach. Honestly, anything to get more people on vim and emacs is a good thing in my book!