Amazon is 40% of online sales in the US, and is probably closer to 100% for some subsets of goods. They effectively have monopoly power and they're using Standard Oil tactics to shake down merchants.
They can use COGS to squeeze margins and even decide which merchants and products they can compete with directly.
When a trillion dollar company is stepping on ten million small merchants, that's robber barron behavior and calls for DOJ intervention.
I think the comment you are responding to means the GP comment, not the story.
IE the retailer trying to estimate their supplier's COGS, not Amazon forcing people to just give it to them.
