As far as I read the support doc, it seems like if say I purchase an ebook from a third party App, should Apple have this data because they mandated the forced use of their payment system. (I don't like user profiles based on purchase data from their store itself as well)

In this example, Apple is not just the store but also your payment processor(a forced one at that). So, they know about which cuts of meat you like and will use that info to target ads when your cobbler pays for some advertisement.

It is not like FB is selling your data, why would they when they earn the most by keeping it to themselves. Quite sure, Apple will do the same thing like they have already started.