If you go to Shalizi's article, which the author links to, then you'll see that polynomial time is a based on very generous assumptions about the problem space

If you go to Shalizi's article, which the author links to, then you'll see that polynomial time is a based on very generous assumptions about the problem space of the economy to make it amenable to convex optimization. Under realistic assumptions, convexity disappears, which pushes economic planning into NP Complete and harder territory.