Just think about what's written there--it refers to Joseph and all his brothers as "that generation." Now 10 of Josephs brothers witnessed events before Joseph's birth and at least 2 of those brothers lived after his death. So they were of a variety of ages but they were contemporaries and viewed as one "generation."
OMG...did you go to the one in TN? I bet you sat near my family, lol. Question about what I've just highlighted: Do we know the age difference of Joseph and all his brothers? I mean, if TEN of his brothers witnessed events before Joseph's birth, is it possible that there was only ten or twelve years in age difference? And the same goes for for the 2 that lived after he died.
I had a family member explain this to me exactly how you described this, OMG. It...just...doesnt...make....sense. And what the heck is a "contemporary"?