You said there was no proof in the Bible that God shortened man’s years to 120. What about Moses in Deuteronomy 34:7, when he died at the age of 120 years?
The life of Moses was divided into 3 periods of 40 years each. First was his time as a prince of Egypt, then his time as a shepherd in Midian, and finally his time as the leader of the Jewish people in the wilderness. Deut. 34:7 hints that Moses didn’t die a natural death but rather his life was shortened because of his disobedience (Numbers 20:9-12). Abraham lived 175 years (Genesis 25:7), Jacob lived 147 years (Genesis 47:28), Joseph lived 110 years (Genesis 50:26), etc. There’s no evidence of a general life expectancy averaging 120 years after the flood.