And if I, a white UK citizen, attacked an immigrant regardless of the immigrant's colour, I wouldn't be racist. According to you.
That might be the worst attempt at a gotcha moment I've ever witnessed. My answer is no, it wouldn't necessarily be racist, it would depend on your motive for attacking them.
So, the Sentinelese aren't racist; they're tribalist.
Is it wrong for the Sentinelese to be tribalist, in your opinion?
Is it wrong for me to be tribalist?
Right and wrong aren't useful terms.
It's not doing the world any favours. War ultimately stems from tribalism, let's not forget. Historical wars over resources stemmed from a tribe's belief that their tribe was more entitled to those resources than the neighbouring tribe, etc. Nothing is truly gained from tribalism. If you think your tribe has better ideas than other tribes, then surely it would be better to invite other tribes in so that they can learn of your ideas. It's idealistic, but without ideals, what do we have to strive for.