Which chemical attack are they talking about? There have been at least 85 seperate documented chemical attacks since the war broke out.
https://www.armscontrol.org/factsheets/Timeline-of-Syrian-Chemical-Weapons-Activity
Absolving the Syrian government of one instance is rather meaningless.
You have inadvertently raised an interesting aspect. The fact that you are unaware of which alleged attack they are talking about demonstrates your lack of any depth of knowledge and your general ignorance about the ongoing UK/US sponsored aggression against Syria.
A recurring theme in all of the other alleged attacks is the dubious role of the "White Helmets". It is they who have provided all of the footage and "evidence" of previous attacks.
The White Helmets, a supposed first responder civil defence organisation, are so obviously a tool of and paid by Western aggressors, specifically the UK, that any "evidence" or "footage" from them is immediately rendered worthless.
Founded in Turkey in 2014 by the now deceased former UK Army captain and obvious UK intel spook James le Mesurier. He died in a fall from a roof in Istanbul in November 2019 where he either fell/was pushed or committed suicide. This happened just as fraud allegations against him and his dodgy UK Intel sponsored White Helmets were becoming problematic.
Also of note when considering the "White Helmets" is the limited area of operation that they have. The Red Crescent(equivalent of the Red Cross) were attacked, murdered and forced out of the areas where the White Helmets operate. In fact the White Helmets operate exclusively in Jihadi held areas (ISIS, Al Nusra, Al Qaeda et al). They have been filmed taking part in ISIS executions and are self evidently UK Intel agents who also play the role of war atrocity propaganda pushers.
There is a lot to say about the White Helmets and the connections with UK Intel including names and dates and incidents.
It is a long and interesting story but it will wait for a couple of hours. I am about to go out for a couple of hours but will happily fill in the many details that put all the other alleged attacks in their proper perspective.