It depends. With conventional AWD (like all Subarus), you are correct that there is significant energy loss due to the AWD because there is a drivetrain to the rear wheels, even if it is not delivering power.
Many hybrids, however, have AWD and they work by having a second, small electric motor that powers the rear wheels only when needed. That means no physical drivetrain to the rear wheels, so no energy loss. That's why a hybrid Toyota RAV4 AWD gets about 15% better gas mileage than the smaller and lighter hybrid Subaru Crosstrek AWD.
The disadvantage of the electric motor approach is that it cannot deliver as much power to the rear wheels as a conventional physical AWD drivetrain can. That said, probably only an issue in deep mud or snow.
Well, no physical drivetrain to the front half of the car would make a difference. But even with electric motors in the rear there is a drivetrain between the wheel and the motor. If the idle rear electric motors are is still spinning then the total mass of spinning parts might well approach the mass of a Subaru drivetrain. Driveshafts are very light.
It sounds like the savings are more in construction costs than fuel savings.