I think you might want to read pages 7–8 again: "Wires must be tensioned to prevent their buckling under load. With tension, wires can support compression loads up to the point where they become slack. The same loads that increase compression in wooden spokes, reduce tension in wires. As in algebra, where negative and positive numbers are combined to give algebraic sums, in spokes tension and compression are the negative and positive forces whose sums depends on built-in spoke tension and the carried load."
So the bottom spokes can support a compressive load because this is smaller than their unloaded tension.
Anyhow, I think we are fundamentally in agreement that all of the spokes in the wheel are in tension at all times. I am not an engineer, but I did run this bit by an engineer. He was in agreement that the fact that the bottom spokes are under less tension does not mean they are supporting the load via compression.
At best, the passage is a bit misleading; none of the spokes on a bicycle wheel are ever in compression (or as Brandt rightly claims, they would buckle). Claiming that the reduced tension (again, correct) supports a "compression load" is misleading.
Sorry I can't stick around for further discussion; I'll check back later.