>Instead, Amazon managers are hiring people they otherwise wouldn't, or shouldn't, just so they can later fire them to hit their goal.
And as OP is correctly pointing out, that can't happen, because the bar raiser can veto any hire they think is unqualified. Or at least it can't happen without the bar raiser colluding - and since the bar raiser is not the hiring manager (or even from the same part of the organization as the hiring manager) and they are not being evaluated on the UAR, they have no incentive for this.
If, as you suggest, managers were hiring _qualified_ people with the intent of firing them... I guess there's no way to prove that isn't happening but FWIW, I'm also at Amazon and have been part of a bunch of interviews and I've seen no evidence of that.