But that's the whole point of the "Hire to Fire" story. The claims being made here are not that managers are firing people to meet quotas, but that they are deliberately sabotaging some of their hires so they have a sacrificial lamb to fire.
You might ask “why would an HM fire someone who is qualified to do the job?” The answer is that Amazon requires that someone must be fired. If you were an HM, who would you rather fire: an existing qualified tenured team member who already knows the ins-and-outs of your team and system, or a new qualified person who would require extensive training and time to get up to the level of the tenured employee? You’re going to pick the new person, even if they are qualified.