Actually no, those are just little easter eggs. Level 21 (the end screen) is secretly a playable level, and can be beaten. Look closely at the edges of the screen. (You can also use the press M, I & Z on the title screen to cheat any unlock all levels, including the secret world)
It's super difficult, so I didn't want to make it feel like a necessary part of the game. But if you're interested... there is more!