This is the crux of it. App Tracking and Transparency is advisory: when you decline tracking, it simply asks the app not to track. There's nothing to force them

This is the crux of it. App Tracking and Transparency is advisory: when you decline tracking, it simply asks the app not to track. There's nothing to force them to comply other than Apple removing apps from the store. It's a similar story to when Apple banned using hardware UUIDs for tracking a few years ago. You still absolutely can do it, it's just that being caught doing so could get you banned from the store.