No evidence given.
The "study" making these allegations is a PowerPoint presentation created by the Foundation for the Study of Democracy, which the report presents as an independent organization. The Foundation is in fact a Russia-based GONGO (government-organized NGO) (p.28) and forms part of the so-called Civic Chamber, established in 2005 to simulate state engagement with civil society.
Although it charges the OPCW with the "juggling of facts" and using "deceitful sources" (p. 1), the study is itself unreferenced and relies overwhelmingly on hearsay and "witness interviews" which may well have not taken place. Grigoriev's previous presentation before the UN, which used the same dubious methodology to charge the White Helmets with organ trafficking, falsely claimed that all cited interviews had been videotaped and published (p. 1), whereas the most recent omits such assurances altogether.
OPCW stated in its report that in Douma in April 2018 chlorine was used. OPCW regretted that it had no access to the dead bodies, which were burnt.
The methodological considerations, activities and timeline are an integral part OPCW's final report on Douma. It does not leave any doubts on the detection of chlorine on Douma: "based on the levels of chlorinated organic derivatives, detected in several environmental samples gathered at the sites of alleged use of toxic chemicals, which are not naturally present in the environment, the FFM concludes that the objects from which the samples were taken at both locations had been in contact with one or more substances containing reactive chlorine" (page 3 of the report).
Recurrent conspiracy narrative on staged chemical attacks in Syria and White Helmets acting as terrorists and also carrying out human organ traffics. The disinformation outlets do not specify what "40 White helmets members" told themselves how they faked chemical evidence, to the Russian investigative team, or how their identity was proven by the investigators. On the website of the Russian "Foundation of the Researches of Issues of Democracy", to which the disinformation outlets refer, the original study is not available.
The tweet of BBC's producer Riam Dalati, mentioned in the claim, doesn't deny the chemical attack; "The producer was expressing his personal opinions about some of the video footage that emerged after the attack but has not claimed that the attack did not happen", the BBC spokesperson told Sputnik.