I don't follow you. Bible isn't written through oral tradition. It's probably written/revised by bunch of people. It needs more assumptions to think that that prediction would reflect what people a few millenia ago thought that to be maximum lifespan. It'd be more interesting to find that information in a legend/ballad etc which are written through oral tradition. Since Bible isn't oral, the author(s) necessarily added their artistic personality to the work (it's possible 120 "made sense" or "sounded good" to the author of that line or is an important number for them etc)