Stack ranking was always intended to be used when a company needs to slim down its workforce and wants to make sure it gets rid of the lowest performers; but the business world cargo-culted it into being used for regular performance reviews when lowering headcount wasn't the goal, a task which it is absolutely awful for.

Business is in the middle of waking up and deciding it's worthless as a general evaluation system; but it's still very good at its intended purpose, and that's what Amazon appears to be using it for here.