The results you cite are counter-intuitive enough that I can't accept them at face value without some confirmation on how learning effects are supposed to spill over equally regardless of geographical location and the amount of communication across firms. Maybe the spillover comes from reading journal articles, people switching jobs between companies, reverse engineering products, and so on, but basically I don't buy it without further evidence. Extraordinary results require extraordinary evidence.
The results you cite are counter-intuitive enough that I can't accept them at face value without some confirmation on how learning effects are supposed to spill over equally regardless of geographical location and the amount of communication across firms. Maybe the spillover comes from reading journal articles, people switching jobs between companies, reverse engineering products, and so on, but basically I don't buy it without further evidence. Extraordinary results require extraordinary evidence.