My uneducated guess is that the primary goal of the latest rhetoric about the possible use of chemical weapons is intended to make this more clear. If "we" (not the US yet, but other countries that are nearer) are signalling a readyness to respond kinetically if chemicals are used, surely the message is that nuclear would trigger an even greater response.
The UK has stated that "all options are in the table" if there is a chemical attack. The implication to me is that a kinetic response would then be a possibility, since everything else is basically being done at this point..
