The natural and proportional response to a tactical nuke is obviously an entire countervalue strike against the US, no military targets just the highest density population of course. Nukes are valuable resources that can't be wasted on military assets.
How is China supposed to know that a tactical nuke isnt to be followed up by a general strike of extermination? Besides, if china is going to go down, I would at least prefer that we gave humanity one last parting gift, which is to restore as much as possible the demographic composition of North America from the pre-columbian era.
Nobod would even ask such a stupid question if such threat was made against the israelis, russian, north korean, pakistanis, americans,why should china act differently.