What a great post on Quora. I've noticed there are a far greater concentration of sensible and intelligent people on Quora than other discussion forums on the internet. But 3000 warheads or thereabouts, the situation is still precarious for China where the US and Russia still have thousands upon thousands more than even this estimate for China. On top of this, both the US and Russia seem to have more long range delivery systems in place. Hypersonic delivery vehicles are good and all but China needs far more DF-41s, JL-3s and HGVs that can reach the entirety of Europe and North America, and cover those massive expanses of land many times over with high yield warheads. Only then can MAD be guaranteed. So far it's just a handful of DF-41s + DF-31s that can achieve this. Maybe also some JL-2s riding on Type 094s. Not good enough when you compare the three.
Maybe, but US cannot plan an all out strike on China without having to also take Russia's arsenal into account. Suppose US can with it's larger nuclear arsenal do a first strike against China while suffering 'acceptable' losses, what has it got left against a Russia that may then use this opportunity to eliminate her own threat? With 300 nukes China may not be able to game it this way. With 3000 it would work.