You should really let go of that folclore because it makes absolutely no sense at all and has absolutely zero connection to the real world.
First of all, it's widely known that Amazon already has a rigid employee structure that pays according to the internal pay scale. This alone shuts dispels the "omg unions keep top performers down", because internally your only shot at a pay increase is to run through their promotion obstacle course.
Unions are about employee representation and negotiating power. They are the only way to create change such as limiting working hours and demand safe working conditions and cut down on arbitrarily firing people for no reason. Without unions you have zero power. With an underperforming union you have some power. Sounds like even a bad union is progress.