If another ecommerce platform wants to compete on price to encourage customers to shop there, they cannot. Any discounts will only go to the seller's bottomline. How is that helping customers?
You are right that this policy is very simple, it's very clear both mathematically and logically. Companies cannot lower their prices lower than their costs. If costs are variable across platforms but prices are fixed to a single value than this will de-facto result in INCREASED prices. If this policy causes someone to leave a platform, that STILL doesn't result in prices any lower than they would absent the rule.
In fact, the sheer existence of the rule is a clear statement from Amazon that they believe they have significant and durable market pricing power, since if they didn't, this policy would clearly lead to sellers and buyers leaving their platform.
In a free market sellers will leave the Amazon ecosystem and move to a platform with less restrictive agreements, resulting in lower consumer prices (albeit with less availability). That’s exactly what GP is arguing. Obviously GP believes in a free market.
Amazon’s belief in their own market power is irrelevant.