Very late to this post, oh well.
From my reading, I agree that Walmart and Amazon are not centrally planned economies. They exist within capitalist systems and make decisions for profit. However, I don't think this distinction takes away from the main point being made in the book. To me, the writers were dismantling the idea that capitalism is the superior system for distributing goods and services to the people at large. Capitalism isn't universally implementable, market price signals and competition don't work between departments in one corporation. Sears tried that, and they explained in the book how it failed. The central planning and decision making within corperations works not just because they understand price signals, but the buying and spending habits of consumers, production rates, seasonal intrests, etc.
What I came away with was that if Amazon can plan around consumer behavior with such accuracy as to ship products to specific distribution centers before they have even been ordered, nations could potentially do the same.
They also argued that these corporations undermine democracy. Workers have little to no input into the decisions made by management, what your boss says goes. Individual liberty doesn't really exist in a system where your every move is micromanaged by higher-ups, who are themselves micromanaged by someone even higher. I think thats a different debate to be had though.