It is the public threat and known capability that matters. ("Nice country you have there. It would be a shame if something happened to it. Now, pay up or something will happen to it".)
They expect us to back down from resisting their military aggressions using the nuclear escalation threat as a plausible excuse. The cabal running the American Government will most likely do just that.
"President Biden" will receive worldwide praise for his "statesmanlike handling" of the crisis in order to cover the abject surrender that it will truly be.
First use of nuclear weapons (this century) will be China vs China in an internal factional dispute.
You internal factional dispute will never happen with nuclear weapons - they are not that stupid.