> but companies like Duolingo or Clever seem to prove a few of the author's arguments wrong.

Except we have no idea whether these companies are actual businesses or not (i.e. have profits). Anectodal - Clever has just as much funding and number of employees as another EdTech business I know about that also interacts with "millions of students" but is barely pushing beyond a few million dollars in revenue. This isn't enough to sustain a business with 100+ employees in the bay area.