The antitrust probe is not strictly regarding the privacy changes themselves, but that Apple did not subject themselves to the same new policy and opt-in UI. They've preferenced their own services and built a multi-billion dollar ad network using data that's dark pattern opted-in for themselves and dark pattern opted-out for third-parties.
In isolation, this may seem like a standard competitive advantage for a platform, but Apple has drawn attention to their own power by wiping out nearly a trillion dollars of public market cap from other tech and ecommerce companies with the change.
Not coincidentally, Apple quietly announced that they are relaxing some of their privacy restrictions during WWDC last week.
