"It's not a monopoly" is exactly the framing. Antitrust isn't inherently concerned with monopolies, it's concerned with market power. It's just that a monopolist has no room to move in claiming that they don't have market power.

They also play this trick where they pretend the only market is "phones" and ignore all of the complementary markets they're tying together with their phones, like app distribution and browser engines etc., since the market definitions for those shouldn't be the same as it is for "phones" when an app store that can be used on Google and Samsung phones can't be used (and therefore doesn't compete) on Apple phones and vice versa.