Because it's a mutation that we can observe in the genetic record. Looking at random mutations over time to that mutation, we can get an approximate date for it's first appearance. Which is exactly what the researchers did in this study.
IOW, this article is opinion.
Nonsense. Out of curiosity, what are your qualifications? Have you read the actual study?
>>Because it's a mutation that we can observe in the genetic record. <<
I wasn't wondering why they called it "a mutation we can observe in the genetic record", I was wondering why they called it a mutation at all.