A lot of people are in a particular tribe because they literally cannot be in the other tribe because the other tribe sees them as subhuman, as people who should be deported, who should lose their rights, etc. A lot of them realize that they're in a tribe and don't particularly like it, but since the political system is set up in a way where you can't reasonably have more than two parties, they don't have a choice.

Basically, the author is making it seem like everyone other than a select few are tribal idiots, but that's a fundamental outcome of our political system. You can pick and choose your policies, but at the end of the day, you're voting for one of two parties.