An interesting distinction between the Volt and other hybrids is that the Volt has an all electric drivetrain. Propulsion is solely from electric motors, while the gas engine acts as a generator.
Other hybrids have electric motors that are better described as power assist. The main propulsion is still a driveshaft connecting engine to the wheels.
As a result, Volt could be easily adapted to other electric power plants should they become feasible. An all battery car, and a future fuel cell car would be obvious extensions. Very little extra engineering would be needed to adapt Volt to the new power supply.
This is the rationale GM uses to call Volt a range extended electric.