If it is, it shouldn't be, and we need some similarly concise term for "action to eliminate biased incentives and promote competition for the good of the individual".
C'mon, this is ridiculous. There is no need to promote competition when competition already exists. Hence a monopoly (or cartel) being a necessary precondition of antitrust action.
Let me explain. I just did a search for "lightning cable" on Amazon. I don't want to pay $19, or whatever it would cost, to buy an Apple branded lightning cable. So I'm going to go for something cheaper. The top result is by Amazon Basics. Then we have YUNSONG, PLmuzsz, and KRISLOG. I have no idea what those are. I have no idea if I'm going to get a steaming pile of inauthentic shit. But with Amazon Basics, I can at least feel confident that they did a reasonable job of finding and vetting a factory to source the product from. And, more importantly, that they have a vested interest in me having a good experience, since Amazon Basics is Amazon.
Generally no, but I don't think a monopoly is a prerequisite for biased incentives to cause consumer harm, particularly for platform companies. Consumers don't have to choose between a bunch of non-monopoly platforms that aren't acting in their best interest when they can collectively decide, via their government, to force the platforms to do as they want.
This is speaking generally - I don't actually have any problem with the specific case of Amazon promoting their AmazonBasics brands.
Antitrust laws specifically address using monopoly power to restrain competition.