Oversimplified. Electric car can act as storage for renewable energy. Moreover charging happens at night when a lot of countries experience oversupply of energy (e.g. in Poland electricity is cheaper during night hours).
I agree that you can make significant savings with G12 tariff in which night usage is about 30% cheaper than during a day in comparison with fixed-price G11. But unless you have your own source of renewable energy your car will be charged with power from coal power plant. So maybe you can charge your car for less but you still use fossil fuel and it doesn't change much in amounts of emitted CO2.