I mean it makes sense at a first principles level.
An ice car connected to a transmission has a lot of losses, additionally, the engine is constantly in and out of various power and efficiency ranges.
Even if you are just hooked up to a generator somewhere else, the generator can run at peak efficiency consistently to charge the battery instead of constantly varying.
You could translate it to any power source. A large wind turbine is going to do better than a small one on top of the car.