I don't think China has the nuclear warhead stockpile to kill 1 billion+ people. Wikipedia says "Current stockpile (usable and not): ~260" which means each would have to kill 3.8 million, which basically means delivering each perfectly into a major city. That's a non-trivial operation to put it mildly.

That said, I don't see any objective (rational or otherwise) of either country that would be advanced by such an attack. And again, we're talking about an attack that will kill 1B+ people, not just any nuclear attack -- that's a different story. Still low probability, but at least there are semi-rational objectives that could motivate it.

https://en.wikipedia.org/wiki/China_and_weapons_of_mass_dest...

Oh, I didn't mean to imply the Chinese had enough stockpiles. I meant they have a big enough economy to rapidly build up their nukes (or invest in bioweapons etc), should they decide to.