Except the Russians have nukes and would use them before losing Siberia.....
But what many people don't realize is that the Chinese feel they were unfairly screwed out of Siberia by the Treaty of Nerchinsk...and really they feel just as strongly they should have Siberia as they do about Taiwan.
The thing is, they have a point. The Czars screwed 'em. My take on Russia's current pro-PRC policy is that Vlad is buying time in the hope that Russia will be better situated 10-20 years hence when the PRC gets full nuclear parity with it. Unfortunately, by then Russian missiles from Chinese ships might have killed thousands of American sailors, which won't help any eventual turn to the West by the Kremlin.