Apple actually does a really good job of making it hard for developers to track a specific device (if they want to get approved for the App Store, that is).
Correlating device type & OS version, IP address and time of access is a pretty good way to connect the dots between “anonymised” IDs. If the same IDs always show up in quick succession from the same IP and device type/version chances are it’s the same person just checking multiple apps on their phone. Time of access and IP address can be used to infer where the person lives (to a certain degree or accuracy), when they are working, etc.
