The fact that it is telling a company that using the app will share data with other companies outside of the one that owns the app is not anti-competitive. It is consumer friendly by exposing shady activity. I actually get explicit requests from Apple about the use of my data in their own apps. It’s only right and expected that I would know the same about anything I buy from them with the expectation of protection.It's anticompetitive because it puts Apple in a privileged position compared to other apps and networks. Apple has access to your demographic info and knows what apps you use, and won't have to display these prompts (or respect them!) in its own apps.
That said, it's a weaker argument than with Google, because Apple itself isn't primarily an advertising network (except for App Store ads) and thus isn't directly competing with the ad networks it's crippling.