Sure cheap labor helps, but is cheap partially due to the currency difference.
Despite anecdotal stories, The US still makes up 20% of world manufacturing and retains its spot as the global leader. http://en.wikipedia.org/wiki/Economy_of_the_United_States
The colocation theory (the theory which the author is talking about) is a good thoery to explain silicon valley though.
If you want to run a tech company, you want it to be colacated near your supply chain (engineers), and other service providers. This is probably a big reason why Silicon Valley has remained dominant despite so many other cities trying.