Managers used to be smarter than their subordinates, so they could steer the work.
With knowledge work, as soon as you turn manager, you're away from the knowledge. That means all of a sudden your subordinates are smarter than you.
So where does that leave the manager? Well, as the article also kind of concludes, the manager is now at the service of the team. Big difference!
Instead of coordinating and guiding the work, the manager now has to make sure the path is clear for the team to do their job. The manager deals with all the bullshit so the team doesn't have to.
This also means that it will be hard for you to notice a great manager, but easy to notice a bad one.