Antitrust is kind of a nebulous smell test (I'll know shit when I smell it kinda thing), and from before the internet era, so I don't think the distinction exists. It's more about how consumers end up interacting with the market due to the actions of the companies being prosecuted/regulated.
The issue in this case would be that Amazon is manipulating the markets involved by effectively being the market. If FooCorp was a minor player in the market, didn't buy up competing marketplaces, and was clear about it's brand as a seller of Foo products that would be fine (think Costco and Kirkland Signature branded products). The issue with Amazon is that it is half of all eCommerce, buys up competing marketplaces to shutter them, it is hellishly difficult to determine what is an Amazon sold product and a marketplace sold product on their site, and their brand is as a marketplace.
The issue in this case would be that Amazon is manipulating the markets involved by effectively being the market. If FooCorp was a minor player in the market, didn't buy up competing marketplaces, and was clear about it's brand as a seller of Foo products that would be fine (think Costco and Kirkland Signature branded products). The issue with Amazon is that it is half of all eCommerce, buys up competing marketplaces to shutter them, it is hellishly difficult to determine what is an Amazon sold product and a marketplace sold product on their site, and their brand is as a marketplace.