It's designed to identify economic regions. The way they do that is to graph all the connections between cities and cut the least-connected ones until 50 clusters emerge.

It doesn't, as you say, mean that someone on the edge of a graph commutes all the way to center---after all the drive time for some of these routes is more than 8 hours.

You could argue that choosing 50 clusters is arbitrary and it would make more sense to severe connections below some threshold "quality" and see how many clusters meet that criteria.