Elon Musk knows something Jeff Bezos doesn’t. Each has had turns as the world’s richest man, and both are media overlords. But whereas Musk’s purchase of Twitter arguably won a presidential election and briefly put the fate of the United States federal government in Musk’s hands, Bezos’s purchase of the Washington Post has bought him nothing but grief. No election victories, no sway in Washington, just the hatred of the journalists he subsidizes to the tune of hundreds of millions of dollars. Media power in the 21st century is about platforms, not publications. Bezos shouldn’t have needed Musk to teach...