Does Competition Really Increase in the Textbook Market When it Becomes Morre Concentrated?

A Vox piece on soaring textbook prices told readers:

"And the college textbook market has changed, too. Publishers used to spread out the cost of a new edition over five years before publishing the next edition and starting the cycle over. Since the publishing industry began consolidating in the 1980s — five major publishers now control 80 percent of the market — competition has become keener, and the window before a new edition has narrowed from five years to three. That means higher prices...