I suggest you read the abstract, at least. The fact that only AT&T had the iPhone back then resulted in a natural experiment: It was only available in certain regions. You can thus compare regions where it was available and where it wasn't, while controlling for "richer people" or "people preferring iPhones".
As a rule of thumb, if you look at something for 3 minutes and have some obvious questions, the scientists that looked at it for several years of their life in great detail might have had those same obvious questions as well.
As a rule of thumb, if you look at something for 3 minutes and have some obvious questions, the scientists that looked at it for several years of their life in great detail might have had those same obvious questions as well.