What do they actually say is the job of the DM?
It's TWO CHAPTERS, or more like, the whole book, so it's a bit difficult to sum up for you, but I'll try (as I'm pretty sure that I know what you're asking).
They say the DM is (and detail each bit with examples): Actor, Director, Improviser, Referee, Storyteller, Teacher, & Worldbuilder. They suggest that a DM ought to "Embrace the Shared Story", "Be Fair & Flexible", "Communicate with Your Players" and that "It's not a competition" and "It's okay to make mistakes".
It goes on to describe that "Every DM is unique" and gives some examples of Playstyles, including "Hack & Slash or Immersive Roleplaying", "All Ages or Mature", "Gritty or Cinematic", "Serious or Silly", "Planned or Improvised", "Heroic or Morally Ambiguous".
It DOES have a chapter on "Ensuring Fun for All", which I
know will cause you concern, but it speaks of "Mutual Respect" and includes a full section on "Respect for the DM", as well as "Respect for the Players", so I don't see it as one-sided as you might fear, but YMMV.