I think an alternative interpretation of the verse offered in the WT was that God gave humans 120 years to repent before he brought the flood
Why did god step down the age of people from almost 1000 to 120 then to 70 or 80.
It used to be a major feature of JW teaching that humanity has progressively degenerated since the fall: in terms of longevity, strength, intelligence, morals.
The old WT literature against evolution relied heavily on the argument that evolution is about the improvement of mankind, and humans are clearly degenerating over the centuries, therefore evolution can't be true. Thus in one fell swoop they managed to misunderstand evolution, the Bible and history.