The counter to that argument is that the brick-and-mortar chains don't claim to be platforms or marketplaces for other products. The business model of a Walmart is pretty simple. Buy wholesale, sell retail.
It's a little more complex than that. Retail chains often charge consumer packaged goods companies for shelf space. Essentially the retailers are selling access to their "platform".
I would not be surprised to see the big retailers passing off a lot of inventory risk to the smaller brands it can push around. Walmart also definitely uses sales data from its stores to sell their own brands (Mainstays and others) alongside other brands.
https://traxretail.com/blog/quick-guide-shelf-space-costs/