That's for the good studies. Let's not pretend that all published studies are honest. Unfortunately it is quite reasonable to be skeptical about extraordinary claims such as this one.
It is reasonable to be skeptical, absolutely. But responding to a study like this with "haha what about if only rich people got iPhones" or "bro don't you know that correlation does not imply causation" is juvenile.
To be fair, their control variables treat the first objection (wealth), not the second (brand preference; and yeah there's some correlation but one doesn't imply the other)