Look up the definition yourself. Singlespeeds are in fact mechanically more efficient than geared bicycles. You are either not thinking about efficiency at all, but about how hard you must work to move a certain distance (this is unrelated to efficiency, which is the amount of energy input that gets turned into useful work), or you are thinking about the biomechanical efficiency of being in a certain gear under certain situations. For example, in order to climb a hill on a singlespeed, it may be necessary to pedal much harder in order to keep moving at a reasonable speed and cadence, which could push you over the anaerobic threshold. Anaerobic, powerful muscle movements are a very inefficient way for your body to turn energy into work. So climbing a hill may cause your metabolism and body to become less efficient for a short period of time, but the bicycle itself is no more or less efficient than it was on the flat before the hill, or will be going down the other side.

It is true that the efficiency gains of a singlespeed (either fixed or free) drivetrain are pretty small, but they do actually exist. I'm a gearophile, but I see no reason to deny what is factually true. The 925 looks like a great bike, by the way. If I could justify the purchase of a singlespeed along with the geared bike I want, I wouldn't hesitate. It looks like a really fun machine!