It's true that they're not working full-time, but I think it's somewhat indicative of how the knowledge and supply chains are spread out. If you were to try to recreate a modern computing infrastructure on a desert island, you'd have to assemble a lot of those people, from the people who know how to smelt ore, to the people who can figure out how to build capacitors, etc.

A wild card that I'm not sure has been tested is how much you could get away with recreating solely from published materials. Given some smart people with a university-library-quality set of books (and let's say a good index), how many of them and how long do you need to go from stone-age to iPhone?