I didn't see the link - can't help you there.
As long as the motor has brushes, then it can operate either as a motor or generator with no modification at all. That's a simplification, it's true, because it ignores the practicalities of what power source will drive it as a generator, the diode needed to prevent the generator from acting like a motor when it isn't being driven, the appropriate speed, and so on. But very basically there's no more to it.