Some (most?) of the black midi scores do cheat a bit. They have bajillions of notes but most of the notes are not audible. Their presence gives cool visual effects (either static if you decide to print it out as classic music notation or dynamic when an app plays the tune) and that's it.
Edit: they typically still have enough audible notes to make it sound like a dozen clones of Chopin played together.
Edit: they typically still have enough audible notes to make it sound like a dozen clones of Chopin played together.