I also have a hard time believing these people have been living on the island for 60,000 years.
The article says they think only 50-400 people live on the island. Don't quote me on this, but I'm pretty sure that isn't enough people to sustain a successful breeding population for humans. If they did reproduce with that few people, they'd eventually go extinct from genetic abnormalities.
I guess there could have been more people in the past, but even then that island is relatively small. How many people could it sustain from natural resources alone. Maybe a few thousand at most. Especially with no agriculture.
I'd wager they're so violent toward offlanders b/c they've trusted outsiders before and got majorly fucked over by them. Either comparatively recently or in their past, and passed down through their collective memory.