So where is the NK and Chinese ground force and air force that numbers hundred thousands or a million on their soil or door-step? Where is the invading hundred thousands? Swim across the Pacific ocean?The problem is that the bombs are needed for retaliatory strike.
Means enough bomb with delivery vehicle has to survive the USA first strike.
And they have to count with the USA missile shield as well.
Means if the USA can destroy 100 Chinese warhead with ABMs, AND china has 500 warhead, AND the USA can destroy 80% of Chinese warheads in first strike then China doesn't has any MAD protection. So, the US can invade North Korea, can declare the independence of Taiwan, can start a war against China mainland, and China can't do anything. For anything less than complete unconditional surrender by China the USA can destroy her in two hours time.
Based on the above I think the next:
1. China increased the number of warheads, the US know it (otherwise what is the point of it? ) , but it was a secret project outside of the known manufacture sites, so nod data in literature.
2. China improved the quality of the warheads by decreasing the required amount of the Pu239, and/or increased the survival ability of them with decoys, or/and hypersonic manoeuvrable weapons.
3. The mixture of above.
Actually, I think it is the no2.
Russia managed it (no2) , hence they are not interested to cancel the START.
Don't just count in your favored preset conclusion, you have to have everything from both side on the table.
Your scenario won't happen in the first place.
P.S. please don't start a "country A nukes country B" war-game scenario, it is going to lead to warning and ban.