One must consider the source, but one must also admit that Assad pulling this stunt at this time would seem to be more counter productive than not.
https://www.rt.com/news/424149-skripal-poisoning-bz-lavrov/
To pull a stunt at a time that would seem to be counter productive can be used to give plausible deniability. Calculating devious megalomaniacs like Assad and Putin would see that at the front of their handbooks.