1. Amazon has attrition metrics.
2. Hiring managers at Amazon are deliberately hiring weak people that they intend to later fire so they can protect the rest of their team.
The argument is that because managers can't actually unilaterally hire weak people that this strategy isn't really feasible.
There are still bad parts of #1, but that isn't the most controversial part of the story.
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.