That's what they have used in the past, and are not bound to it. If you read any of Lina Khan's work, it's clear that they'll take a more holistic view of the impact of lack of competition.
That's a good point, it's important to note the current FTC chair is working to change the standards that are used. I'm excited to see how that pans out. I think the lack of antitrust enforcement in recent decades is a really underrated problem in American governance.