Just that, present day memory is arranged in 2d surface, not in 3d volume. And there are issues like cooling, so we don't know if r^3 scaling memories will happen in the future.
Well, SSDs are 3d planes of 2d chips now, so we're on our way.
http://www.intel.com/content/www/us/en/solid-state-drives/3d...
Part two is much better and is based on sound principles. The only complaint there is that it's a bit misleading; if you arrange your memory in a sphere, you get access times as the cube root of N for a very long time before you start running into information-theoretical limits based on the area of the memory region.