However much tribalism is a natural instinct and may have served some useful purpose in the past, its pretty clear its not immutable. People can work at minimizing or even eliminating this "natural" tendency, making the effort to get to know and understand "the other", to remind themselves that "different" is not "less", "wrong" or "unacceptable in my society".
Bigots do not do this. Instead they focus and build on anything that confirms their dislike of those who are different. They minimize any indication that "the other" might have any redeeming quality. They set "the other" as a class apart, assuming, and even insisting, that the majority are less honest, trustworthy, intelligent and moral than one's own group. In the bigot's mind the essential humanity of "the other" is reduced so they may justify their own hatred.
Tribalism might be instinctive, but its a weak instinct and people, as individuals and societies, have overcome that instinct throughout history. Bigotry is ultimately a choice people and nations make to embrace and build on that tribal instinct, to hate those who are different, to lead bigots towards individual and systemic bias against "the other", individual and legalized violence against them and if left unchecked, to genocide.
"Instinctive" is not automatically "acceptable", and using that as justification for one's dislike of those who are different is a failed argument.