How does David Smith support his claim that ""All cities today are world cities?""

In complete sentences.

There is some truth to the claim that, "All cities today are world cities". All the emerging cities of Africa, Latin America, and Southeast Asia are linked to the global economy and, particularly, to the network of world cities, in a variety of ways. But it is also clear that various metropolises fit into the world urban hierarchy at different levels and play very distinctive roles in that wider system. Analysis of urban systems, whether global, national, or regional, from the perspective of geography or sociology, using the old human ecological framework or the newer urban political economy approach, emphasizes power and dominance. Furthermore, one of the key advantages of identifying where places fit into positions in these systems, is that "structural isomorphism" will lead to similar roles. In other words, if two cites are at or about the same level in the  urban hierarchy, we should logically expect them to follow broadly similar dynamics. In the 1970s and 1980s, at a time when the neo-Marxist world-system scholarship was blossoming, an "urbanization in the world-economy" approach emerged. This perspective took its initial impetus from Manuel Castells’ suggestion that we should consider the growth of third world cities as “dependent urbanization.