Wal-Mart putting their own brand of toilet paper on more prominent end cap shelves is fundamentally different than Amazon promoting their own Amazon Basics toilet paper on a search results page on Amazon.com. Amazon has a much greater ability to drive conversion to its own products than a brick-and-mortar retailer (e.g. limited screen space, UI, information hierarchy, better data, etc.)
Thompson's argument that "it's all the same," "private labeling has been going on forever," is flimsy and intellectually lazy.
Once we agree on this basic set of facts the more interesting question becomes: is this dynamic bad for consumers long-term?
The crux of this round of tech anti-trust scrutiny is time horizon. Many of these practices are neutral or beneficial to consumers in the short-term, but that's always how monopolies operate. Undercutting prices to put a competitor out of business benefit consumers immediately, but hurt consumers after the competitor goes out of business and the monopolist can raise prices. Amazon selling Amazon Basics products at a loss or near-loss is good for customers now, but if it puts too many other online retailers out of business, eventually they'll raise prices and hurt consumers.