Foreigners having been there isn’t the claim. They kill many of the people that show up in short order. What is claimed is that they are so isolated that they have maintained a Stone Age culture to this day and fight so fiercely against outside interference that the Japanese government decided to protect them for their own sake but also for the sake others. I can’t say what their ancestry is but they definitely live an extremely unusual and isolated lifestyle to this day. It is a very unusual case of isolates.
You can read all about them but little is known because they are unapproachable and they can’t speak in any common language. If you know more about their heritage than I do or anyone else in anthropology please share it because there are lots of people that would love to know.
Here is a video of a group trying to approach them by boat:
