Is China really going to nuke the US because the US says that it officially no longer supports the one China policy?

Sure, they bluster. But that would be really stupid. "You said something I don't like, so I'm going to kill us both" level of stupid. China talks tough, but I don't think they're actually that stupid.

> Technically the US supports the one China policy. It is assumed WW3 starts if that ever changes.

The US walks a line on Taiwan that falls far short of actual support for One China but does enough to support it that the US has a lot of room to escalate with China if the US wants to without starting WW3.