That thing you're told happens 2 decades after the game ends... How are you told that? Do you have to sit through all the credits to see it? I played through, got a little bit of dialogue at the very end from shivers and esprit du corps indicating that something is going to happen next, and then painfully long unstoppable credits. Tried to wait the credits out but it got to be too much so I ended up alt+F4ing and started a new run to try a different build.
It's in the church, you need specific high stats while getting everyone to dance. You pass out, speak with the city, and if you have the correct stats you receive a vision of the nuke.