Just cause it is a sailing vessel does not mean it does not have an engine. It probably uses a couple of 100 liters of disel to get in and out of port, driving generators and heating the boat.

On top of that the sails will degrade with time and the ship will need to be repaired and there is allot more crew and manpower needed per unit of cargo.

Everything has environmental cost in CO2 emissions, and the amount of useful work you get from a large container ship per amount CO2 emissions is insane. The only thing that can get you those kinds of numbers is scale.