Seems I have fallen for a hoax. Over the years, tesla charging stations appeared here and their. Some of them with a suspicious container. People told me that there is a backup generator in there, when the power grid fails to provide. But the generator /container ist to small to provide the output needed for supercharging. But I couldn't find any article explaining the real purpose.
A container/ small building could be the inverter/transformer for DC fast charging. Essentially most things for DC fast charging are handled in this small ~10m^2 room and the charging stations that you can see provide the plug and the interface for the customer and nothing else. Everything needed to get grid AC turned into 300V or 600V DC is in the box.
Even a diesel generator charging an EV is less pollutant than an ICE car.
It is basically the concept of a hybrid.
Combustion engines need to rotate at a specific rpm to be efficient. That is why ICE cars have gearboxes, because otherwise they would be even less energy efficient.