[lurquer]

It had nothing to do with lifespans.

The pronouncement related to the Flood: it occurred 120 years after the pronouncement.

There’s some confusion in the chronology, due to Noah’s genealogy, age, and the births of his sons being kinda garbled up in the same chapter. (That is, some might say the flood happened 100 years after the pronouncement instead of 120.)

But no fair reading would assume it has to do with lifespans as even in the authors time (Moses, presumably) people were living longer than 120 years. For instance, Sara lived to 127, Abraham lived to 175, and Jacob made it to 140 or so.