Syria could have easily synthesized new Sarin after handing over their chemical weapons.

[DetlefK]

https://www.wired.com/2017/04/syria-sarin-chemical-weapons-chlorine/

* You need a well-stocked chemical lab of about 200 square-feet (~20 square-meters). That can be hidden in any warehouse. And is impossible to detect unless you walk in.
* You need a decent chemicist.
* The synthesis takes several days.

Getting rid of chemical weapons means nothing when it's so easy to synthesize new ones.

How easy?
https://en.wikipedia.org/wiki/Aum_Shinrikyo
The Aum Shinrikyo was a japanese doomsday-cult who conducted several terror-attacks in the 1990s. Two of them were with Sarin, killing 20 people and hurting more than 6000 people to varying degrees.