Why Kishi, why? It was already clear Tsunade couldn't have known the 1st Hokage, based on the timeline. Now, suddenly she did, so we get a funny joke... doesn't he have staff to point this stuff out to him before he does it?

After all, the 1st and 2nd Hokage reigned for a not inconsiderable period, so even if the 1st knew her just before his death, and the 2nd reigned the minimal number of years, she's still too old to be a genin under (adult) Saru, with his beard and such. Annoying.