How did Kenji know? (Regarding end of 21st Century Boys)

I'd been spoiled to Katsumata being adult Friend the entire time and Fukube dying in 1972 long ago, so as I was reading the manga I was able to parse all of the hints and piece the logic on how it works together. I don't think I have any issues with it, and I think it may actually make more sense than Fukube living past childhood. I seriously suspect it was Urasawa's intent all along, and he was just too vague about it in the original release.

What I am puzzled about, though, is how Kenji knew Fukube died in 1972. He is the source we learn this information from, but...how does he know??? Is he speculating? Just pulling it out of his butt? Why would he speculate that? That's a really specific theory to speculate.

Could he have conceivably learned this information before we see him return, and he was just holding onto it because it wouldn't really matter if he'd told anyone? When everyone else already knew that current Friend was not Fukube. Does anything contradict this? Because that's the only thing I can think of.