Well, OK. That just proves he didn't need 70 to "forever less one day" in order to write it.
But the evidence is strongly on the side that he needed at least 50 years. I mean, did you see him writing Gatsby before the 1909 copyright act? No, you didn't, because before that, he'd have somewhere around 40 piddling years of copyright protection. But the 1909 act passes and sure enough, Gatsby comes out less than 15 years later.