"Startup"? This looks more like a "business". Cheers to them for staying low-pro & avoiding the hypebeast for all this time.
tl;dr sheet music on iPad. There's a $20 billion music buyer market in the US and $80bn overseas, but most of that money is spent on instruments. The market segments Chromatik is chasing are a) classical musicians, of whom there are few (but enough) and b) high school band musicians, of whom there are plenty. The crossover with computer-based musicians is almost non-existent. Outside of the classical sphere hardly any working musicians use sheet music.
Realistically the number of people who would like to learn an instrument is astronomically high. Most people try, get frustrated, and quit. If Chromatik makes learning far more enjoyable and simple, they could easily grow the size of this market.