The study talks about other pollutants (like Sulfur Oxide and other particulates), but not CO2. The amount of CO2 emited by these ships is not stated in the article, but can be estimated from their rate of fuel consumption. That said, in general ships are more efficient than cars, especially at this scale. It's entirely likely that the CO2 per mass of freight per distance covered by container ship is of the order of magnitude, or lower than in a car.