That's a nice story, but I think restricting yourself to exactly one teaching methods is needlessly limited.

Yes, you probably need a proper textbook and (ideally) a teacher to learn grammar and the language rules. This is hard work, but IMO gamified apps make users a huge disservice by handwaving this and hoping the user magically figures it all out. But, like the author found out, grammar alone won't make you fluent.

I'm personally very fond of flashcards (Anki). Yes, memorizing words is just a part of language learning, but it's important and FSRS is extremely good at it. Way better than repeatedly reading a textbook.

I personally hate duolingo for many reasons (it doesn't work for me), but some of my friends use it. This touches another important thing: regularity. Gamified apps and flashcards make it easy to form a habit. You can complete your daily lesson in a bus. And they are (more) fun. Even ineffective learning method is better than nothing.

Finally, ymmv and there's no one size fits all. I got pretty good (fluent and communicative) by in Russian by initially just studying flashcards (followed by reading and listening - another very important component) - because grammar is similar to my native (Slavic) language and I could, actually, figure out most of it. Textbooks came much later. It was not as easy with German...

PS. worth noting that the author explicitly says that this app is meant to teach you just the very basics and numerals, not for language learning