They own a majority of the battery company. They have been working on this for many years. They plan to build in bulk (the plant they are making in Tenn will produce 200,000 batteries a year). All this helps Nissan keep the costs down.
Now they likely won't have enough capacity to help other car companies, but they will have enough to keep their costs down.
And hopefully they will help start a bit of a battery war and get others to cut their costs too...
ps....we are already talking about this in the Battery Forum