Earlier quoted context omitted.
did you read it all? it claims the prices are higher due to the hidden tax and that amazon is price fixing by threatening to block 3rd party sellers if they sell at a more reasonable price elsewhere. this doesnt necessarily mean that consumers will get a better deal elsewhere but it is probably unhealthy to put all that price setting behaviour in the hands of one company.
That doesn't really sound like price setting power. They are saying you can set whatever price you want, but it has to be the same as other sales channels.
In this hypothetical example with demonstration numbers for effect, you could sell your cup for a minimum of $1.50 on Bmazon and $3 on Amazon - everything above that is pure profit. In such a scenario, you would obviously much prefer selling at $2.50 on Bmazon over selling for $3.50 on Amazon, since you make 2x the profit, and the average customer would much prefer to buy the device at a ~30% discount! Unless the customer legitimately derives an extra $1 worth of value from using Amazon instead of Bmazon, in which case Amazon gets the sale anyway.
But, if 90% of your sales are on Amazon, then you can't offer this deal that both you and the customer are legitimately incentivized to do, because you'd lose 80% of your revenue.
In such a scenario, Amazon has no competitive incentive to reduce their fees! It suppresses market signals towards lower-overhead sales platforms, i.e. you have no way to signal to your customers that a sale on Bmazon benefits you twice as much as on Amazon.
Basically, Amazon is trying to abuse a network-effect instead of actually competing with their competition. They're deplatforming anyone who doesn't voluntarily price-fix for them. It's insane.