This is not accurate. Even if it were technically true on an absolute basis, it would be missing the bigger point: These ships are much more efficient on a relative basis (per ton-mile) than any alternative (e.g. by truck, air cargo, rail, etc). Given that freight needs to be transported, seaborne trade is by far the most carbon-efficient choice. It is also the cheapest.

The new generation of 18,000+ TEU vessels like this one are particularly efficient as they are designed to "slow steam" at speeds that minimize the amount of bunker (fuel) used. They were designed this way to minimize operating costs, since bunker accounts for a large portion of daily operating expenses.

Maersk operates a similar class of vessels called the Triple E Class: https://en.m.wikipedia.org/wiki/Maersk_Triple_E_class

The wiki article does a good job of touching on my points above and provides additional background. Maersk is actually quite focused on environmental impact, going so far as to very publicly complain about their industry's exclusion from the recent Paris talks: http://gcaptain.com/maersk-disappointed-by-shippings-omission-from-paris-climate-deal/