I think the nuclear thing is basically a Nothingburger. They can't just flatten Ukraine, since the whole point of the war is to conquer Ukraine. And they're not crazy enough to aim at Berlin, Warsau, or Paris.

And we actually don't know what Biden and Trump discussed. Maybe they decided to play good cop / bad cop. This really does open the possibility of ending the war within 24 hours of Trump's inauguration.

Would explain why Biden was suddenly so cheerful.