Privacy is not all-or-nothing, like a switch. Apple has been playing a cat-and-mouse game with ad companies in which they provide a token for advertisers in exchange for disallowing fingerprinting, and then ad companies abuse that, so they provide a different rotating token and do more to disallow fingerprinting, and then ad companies abuse that, so they clamp down even more, and then ad companies abuse something else, so now they're clamping down even more, and presumably ad companies will figure out still more ways to abuse something, and so on.

There is no such thing as a third-party guarantee that doesn't also disallow useful functionality.