By this logic anything would be made of squares, because it's rendered on a monitor.
I think what they mean, is that an individual "thing" should be just a geometric shape or a small collection of simple shapes. That's like the most basic understanding of the diversifier to me. For example, a character is a square, a bullet is a circle and so on. I think making a combination (like a humanoid made of shapes) is still valid as long as each part represented as a shape has a purpose for the game (e.g. circle head, square torso, rectangle limbs, as long as each part can move independently to a degree).
They also allow more complex shapes, but say it's not ideal.