Acshually a protective mutation was quickly being selected for (more precisely, the lack of it was strongly selected against).

Paper about Kuru and mutations referenced in comment here: https://news.ycombinator.com/item?id=49719102

The paper relies on the fact that people who were dying did not have the gene, but those that didn't ... did

That's not really enough to say "We have found the gene" - it's just really good data to warrant further investigation

Also, the incubation period of the disease is up to 50 odd years, have there been follow up studies?