I never thought about it before - but seeing that chart of "Megaregions", almost none of my work travel within the US from the last 15 years has been outside of those regions.
But more to your point, there are plenty of other sizable cities that are not part of the megaregions the author highlights, and I would agree that those cities are less dynamic and less engines of economic growth due to their isolation - places like Phoenix, Salt Lake City, Minneapolis, etc. The only difference I think is that it "feels" to me like there is an emerging megaregion on the eastern slope of the Front Range in Colorado, i.e. Ft. Collins-Boulder-Denver-Colorado Springs area.
However that someone in Dallas is more likely to choose a supplier in Austin than one in Boston is interesting - once you leave Dallas does the extra distance to Boston mean anything? I can come up with all kinds of weird ideas.