The other problem is that their learning technique, in which you start with fewer notes and add in all the notes over time, is not good at ALL. It masks certain things from you and even makes some things harder at lower difficulties when it's playing some scale and you don't know that so you're just playing random notes in it. Knowing that it was just a pentatonic scale or something would make it much simpler. Instead, you're taught to just play isolated notes instead of learning how to understand what that scale was, how to play it, and how to apply that to other songs. It's almost outright hostile towards a big picture music theory based approach on teaching how to play and instead focuses on memorizing the specific notes for each section of each song.

Anyway, piano scales and chords are not challenging to learn compared to most piano technique. If you don't have a teacher then buy Hanon, get a decent music theory book, and look up Youtube videos. Any of these training wheels based learning approaches seem to just assume that once you do it enough you'll pick up the theory. No. If you can't afford lessons (which I stress are very important) then you should at least make use of simple music theory books and videos on Youtube.