> The length of our days is generally seventy years, or eighty years if one is strong

Which translation is that? Or are you paraphrasing?

Most translations don't include "generally," and therefore read more as an upper bound "if one is strong" than an average.

Just a poor memory translation. Yeah, generally is incorrect - though I think the correct phrasing also implies an average age of natural death, rather than an upper bound. There were certainly plenty of people living past 80. In the aforementioned study of Ancient Greeks, there were at least 3 centurions - Aristarchos, Democritos, and Gorgias. Granted 1400BC is a thousand years yet prior to that already ancient time, but life peaks seem to be relatively unmoving for humans, and so I don't see any major reason to think there would have been a major difference between 400BC and 1400BC.