They're not saying that. They're assuming that Amazon (or the seller on their platform) buys at a wholesale price below what the manufacturer charges as retail

They're not saying that. They're assuming that Amazon (or the seller on their platform) buys at a wholesale price below what the manufacturer charges as retail on their own website. So the manufacturer loses out on the difference if you buy from Amazon instead of direct. That's broadly going to be true whether the Amazon retail price ends up being higher or lower than the manufacturer's retail price.