I've spent a huge portion of my life learning how humans learn languages and how to teach them better. I spent my early adulthood in Taiwan where I became fluent speaker and pretty comfortable reader, too. The approach this site uses is
far superior to the flashcard vocab word-driven approach taken by duolingo, memrise and so many others.
In my experience, at the beginner level the most important thing is getting a good grasp on phonics and learning some vocabulary. But once a student is even lower intermediate it's a way better to spend time doing Extensive Reading instead of vocab drills. Extensive Reading—reading that is easy enough no dictionary is needed—is still great for building vocabulary, and it teaches colocations, grammar and the target language's culture, too. As ER builds up vocabulary, more and more radio and podcasts become comprehensible. In the long run, lots and lots of input (both reading and listening) and opportunities to use the language extemporaneously in real life is pretty much assured to lead to full bilingual proficiency.
The interesting thing about this app, is it just might allow students to transition from more formal study and into Extensive Reading at an earlier point than they otherwise would. The risk is that students would continue choosing books with a lot of words they don't understand and rob themselves of actually "reading" in its natural sense.