Chapter eleven gives three examples of the same thing and lets the reader draw the conclusion.
Thirty spokes meet at a hub, and what makes the cart usable is the hole in the middle. Clay is worked into a pot, and what makes the pot usable is the space inside it. Doors and windows are cut into a wall to make a room, and what makes the room usable is the part that is not there.
The examples are chosen with care. A wheel hub, a pot and a doorway are the most ordinary objects in a pre-industrial household, and in each the absence is not a by-product of making the thing but the entire purpose of making it. Nobody wants a solid pot. The craftsman's skill in all three cases consists of arranging material around a void so that the void becomes usable.
This is the concrete version of the argument chapters four, five and six make in abstract terms about vessels, bellows and valleys. Here it is not a claim about the cosmos but about carpentry, and it can be verified without accepting anything.
What the chapter never says is that the material is unimportant. There has to be a hub for there to be a hole in it. The final line is a balance rather than a preference, and it is the balance, not the paradox, that has kept the chapter in circulation.