Either a) The Person who wrote this article doesn't understand this policy or b) Linden Homes don't.
Code Level 6 and Zero carbon are two different things. Code Level 6 takes into account all sorts of things including water efficiency and materials. Zero Carbon is just energy use within the home.
It costs way more to retrofit a house to be low energy than to build it correctly in the first place. New housebuilding needs to be continued to be pushed.