One of the defining aspects of old-school games is the ability to teach through gameplay. Since there wasn’t a lot of storage space for text to explain individual moves, games had to organically lead players down the correct path. This took the shape of things like placing a single Goomba in Super Mario Bros. or the introductory sequence of Mega Man X.
Indie games tend to follow that design philosophy, either out of choice for retro aesthetics or due to smaller development teams (sometimes a single person). The goal is to teach solely with gameplay and eschew standardized tutorial levels that most players tend to hate. Circles definitely follows that lead, not relying on simple things like words to teach you everything you need to know to succeed.
It truly is wonderfully designed, even if the game is a bit short.