How can an all flying rudder be almost 100% more efficient than a vertical stabilator and rudder?
Because on the Zenith the massively slab sided fuselage is the vertical stabilizer, and the rudder is still the rudder.  The surface area of the all flying rudder is about the same as the rudder on a normal plane, and it depends on the fuselage for weathervaning tendency.   There's no free lunch.  If you build a Zenith with a skinny boom tail and the all flying rudder, it will be quite marginally stable in yaw, like a Fokker DR-1.