Incredibly naive or incredibly aware. I'm just stating the simple absurdity of your assumptions, Russia has more to gain from a world without China than the U.S. does, if anything, the eradication of China would only mean less competition for Russia and the U.S., to the former who's a superpower, and to the latter, is not even a superpower.
A plausible explanation for China's lack of nuclear expansion would be that they couldn't do it. The Chinese economy didn't really do anything until the 70s and 80s, and even then, proliferating nuclear weapons would of costed them billions of dollars, which would of wasted money that could of been spent elsewhere. Besides from that, it is highly probable that China did not need it's own Nuclear Triad, as they were protected by the Soviet Union until their split.
It's simple really. Just because their economy is ruined does not mean Russia will be "lorded" by the West for the next few decades. In fact, a limited nuclear retaliation of 100-200 nukes would pretty much wipe China off the map. That'll leave about ~1000 nukes for Russia, which in itself, acts as a political card. I.E. "help us rebuild or f*ck this!" It's simply more advantageous for the Russians to not destroy the West (or even the World) and just leave China doomed and Russia crippled.
