This is still pretty big, though. I imagine further legislation will be easier if the negotiators can say that legally nobody has any chemical weapons right now.
Of course, I'm sure there's facilities that could start making them in a hurry.
Also, the world at least claiming there aren’t any may increase the political costs of their use. Any country that wants to use them will face the backlash or being the “only one” to have them, even if it may not technically be the truth.
Honestly I wouldn't be surprised if the big players have replaced chemical weapons with something just as horrible but more "smart" like micro-drones with explosive charges and facial recognition.