From the 2-spoke website: "The benefits of this wheel in a one hour time trial compared to a standard carbon 3-spoke wheel can be as much as 2.5 minutes (!) when averaging 51 km/h."

2.5 minutes over an hour means an increase of speed of a bit more than 4%. Because of the cubic nature of aero drag, that would mean a decrease in CdA of ~12% compared to a 3-spoke wheel. That's not a 12% decrease in the wheel's CdA -- they're claiming a 12% decrease in total rider + bike CdA.