Out of curiosity, how does the app make you enjoy the process? I took a quick look and it seems to have the same basic progression that most classes have, I assume the paid courses are different and better but I can't see them.
When I had piano lessons as a child, playing classical music for years became boring and uninteresting to me. I believe apps with a wide range of music help keep interest levels up. The app seems like a great way to dip your toes into playing piano without committing to finding a teacher.
For me, it was just "hitting a few symbols". Suddenly in 2 weeks, I was reading music. It uses exactly the same tactics as any videogame lets you figure out game mechanics.
Suddenly, you're playing another state of the game...and oh hey it's THAT song.
This is hard to conceptualize for someone who can already play. But you have to understand that in October, I had no clue I would be reading music in December.