Re: So what happens....
I believe you may have confused Apple's differential privacy with something they implemented in OSX Safari a few years ago.
It was the the feature in Safari that would make advertisers believe you had visited sites that you hadn't - presumably websites drawn from a pre-compiled whitelist (so no KinkyStuff.com or ISISareCool.org).
Differential Privacy is different, so take a few minutes to scan https://en.wikipedia.org/wiki/Differential_privacy
At this stage, exactly how Apple will implement it is not known, but the concept is that Apple will have data about all their users, but can't reverse engineer that data (because of maths) to identify anything about an individual user.
Of course it goes without saying that the implementation key.