Gen. 6:3 says, "And the Lord said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years."
Gen. 5:32 records, "And Noah was five hundred years old: and Noah begat Shem, Ham, and Japheth." Following the record of Noah's birth, and the birth of his sons at age 500, we see the declaration of the Lord that He will destroy mankind because of their wickedness and corruption, including the crossbreeding of fallen angels with women, thus, producing giants, men of great stature and strength.
In Gen. 7:5-6 we read, "And Noah did according to all that the Lord commanded him. And Noah was SIX HUNDRED YEARS OLD when the flood of waters was upon the earth."
God decreed 120 years as the limit to man's time, but Noah only knew about it a good deal less than 100 years. Still, WTS can focus on the decree. They can even say how Russell/Rutherford didn't know the full limit of what they were doing at the beginning of the decree to limit "the last days" as a parallel.