Good question on the low power of motors. Most of the low cost pumps have often DC brushed motors which do not need a controller to steer but uses a brush. Once there is not enough power (in the morning and evening) I think there will be a point where the power will be too low to overcome the torque and the motor stops. The centrifugal pump can still rotate easily compared to other types so I would expect the startup current will be low as well. Panels will deliver a constant voltage while the current will drop when light conditions are dropping. I think the simplest is to search for these aQuarium pumps (24/7 performance) on 12v but I doubt many cars/trucks have built in fishtanks ^^)
Indeed, the man is painting the grass green.