OK--I see your point. But in Africa, they take the high level of diversity as meaning that they are the oldest populations, but if as you say the Melanesians are the farthest from the Africans, it does not make sense that the Melanesians would be a very old line.
I was told that Africa has more genetic diversity than the rest of the world combined. I guess Melanesia must be second. If you believe in molecular clocks, than the oldest population will have the DNA that the daughter populations will have variants of. Since it takes time to accumulate mutations, the more recent populations splits will have fewer variants and the older population splits will have more. Melanesians have more therefore they're older.