Where does the supply come from? You could still argue that people choose this as a career when they have the choice of better-paid ones, increasing the supply of people with the smarts and training needed by the publishers.
I don't know if that's what's happening, but it might work towards TFA's point.
Most people don't have the "choice" of being an engineer or software developer currently.
To be blunt, it's much easier for the majority of the population to get an English degree or some other generic liberal arts degree and therefore be qualified for an entry level job in the publishing industry.
I'm sure someone somewhere is giving up a highly lucrative job to roll the dice on the next great American novel, but it's not a meaningful number.
It probably has to do with the fact that we condition children and adolescents to consider white-collar jobs as more noble than blue-collar jobs, then we tell them that to get a good white-collar job, they need a degree... and then we make STEM degrees hard by subjecting students to more math than most people realistically need. So we have a lot of frontend developers who know calculus and an oversupply of people with humanities degrees.
With that degree, you're generally pushed toward jobs in journalism, publishing, graphic design, teaching, administrative functions, and so on. Most of these pay relatively little.