He works with the Santa Fe Institute that specializes in studying complex adaptive systems. What all CAS have in common is the organic growth patterns that seem endemic to biological organisms. Also, if you consider the fundamental building block of the city to be its people rather than the physical infrastructure, then you are dealing with a biological system. This is no different than looking at an ant colony as a superorganism - yeah they have some dirt and tunnels and some rocks, but for the most part the important factor in the colony is the mass of individual ants.