While that's certainly an aspect, "cool" people can be identified by others even in the in-group. Usually it's related to them having genuine authenticity vs a "poser" who is distinctly uncool.

Sure, there is social hierarchy within the tribe. But the tribe members all share a common culture. The phone isn't necessarily about being cool, it's about conforming to the rest of the tribe. A tribe that all uses Android is possible, but the interlocking nature of social networks combined with a teen's heightened sensitivity to being an outsider creates a stronger "winner-take-all" mechanism in teen social networks compared to adults.

a group of 87% is no longer a "group" in my opinion. It is "the rest".

The group isn't defined by the phone, it's defined by the relationships. The phone is just along for the ride. Think of it as an aggressive virus that displaces weaker ones in the social network.