I had a Volt. It only had one
I had a Volt. It only had one power train. Only the electric motor provides power to propel the car forward. The gasoline motor only functioned as a generator for the high voltage battery. It was not connected to any axle. The author may be confusing the Volt with the much more complicated Toyota Prius or some other similar hybrids.
The Volt was totally unique in that it was totally electric for the first 40-60 miles and then the gasoline powered generator kicked in to eliminate range anxiety and to allow for longer trips.
If a driver lived within twenty miles or so of his workplace, he could go for weeks or months without ever using gasoline and then he could go visit relatives a thousand miles away without having to rent a car or own a second car with an internal combustion engine.
The Volt was technically a hybrid because it did have a gas tank. But it was different than all the other hybrids which really do have two power trains and switch back and forth between gas and electric depending on the task of the moment, going uphill, down hill, cruising down a highway, stopping and starting in city traffic etc.
The Volt was always being propelled forward by the electric motor at all times. On longer trips, the gas generator would kick in but only to provide electricity for the electric motor which was always driving the car.