While I do believe this hasn't really happened, surely there's a strong observational bias involved ("AFAIK" - but why would you know? :) If they're isolated, chances of learning about them would be very slim by definition)
It's not a well controlled experiment, but it's data. Even in that minimalist scenario they have some tools. Because it's just obvious that if you had no tools your odds of survival are pretty frickin' low. People die in the wilderness all the time even with tools.
