As of March 8, 2022, China has no claims on Russian land, but history shows that if a juicy opportunity turns up, countries start to come up with claims that were previously forgotten.
Yes, this scenario is unlikely. But not beyond reasonable doubt. Multinational empires have imploded and disintegrated before (Tsarist Russia, Austria-Hungary, the Ottoman Empire), and a lost war was the usual cause.
I guess that a potential takeover of the territory would look more like the takeover of Tibet in 1950. A fait accompli executed on a very short timeline.
All this talk about taking over Russian territory is just pure fantasy. Russia, on a liberal / anti-imperialist wave, even in a bankrupt, extremely weakened state could only become kind enough to detach Yakutia, Caucasus, and some other distinctly ethnically different autonomous regions, but it won't allow anything to happen to the ethnically Russian regions, nor these regions themselves could imaginably see themselves separate. Russia would still be a nuclear power, and I doubt the Chinese would risk a world apocalypse for a piece of tundra.
Could you provide some links? I would like to learn more about that. As far as I knew until just now the treaties between russia and china aren't all that old and the PRC has no claims on any currently russian land. But I am not well informed about the topic at all.
(After some research: The ROC and freer press were quite unhappy with the treaty and ROC still formally claims those areas if I understand it correctly.)
Overall it is an interesting (albeit very unlikely) scenario, maybe because it reminds me so much of old EU4 matches I had.