To understand Russia's claims, you need to read where they make claims.
Russia did predict a chlorine false flag attack back in March:
https://www.rt.com/news/421589-us-preparing-syria-provocations-airstrikes/
https://www.rt.com/news/421753-syria-ghouta-chemical-provocation/
However, since the claim of an attack on the 7th they have always claimed that there were no chemicals used at all, but rather, it was all fabrications. Their earliest supports for this claim before they even sent a specialist team to investigate it were the teams they had already present sorting out an evacuation plan with the rebels at the time:
https://www.rt.com/news/423520-jaysh-islam-withdrawal-talks-douma/
While they had their own investigative teams deploy the fastest, they got a quick response from the OPCW to send a team out, getting a confirmation on Monday (although they did not actually fly out to them until the ceasefire evacuation was completed due to barricades and mines the rebels had laid):
https://www.rt.com/news/423732-opwc-sends-syria-chemical-probe/
What they accused the rebels of was using propaganda tactics taught to them. With the earlier predictions, they said Americans had taught them, but after further investigation they are saying the UK not only trained them but specifically instructed them on how and when to fake this attack:
https://www.rt.com/news/424047-russian-mod-syria-statement/
Earlier they debunked the footage from an indoor school scene as having been taken 5 years ago (already posted that link), and now they are saying the night footage of people being sprayed with water was the result of non-doctor provocateurs manipulating people at a hospital into a panic after a bombing had taken place (the footage does very clearly show a post-bombing scene, but not gas, yet this does not rule out gas having been present).
Red Crescent (like the Red Cross over there) said they encountered no chemical victims. The WHO was saying 500 people were harmed by it (restating White Helmets claims) but have provided no evidence after Russia asked them to name any of the organizational partners or hospitals (there was only one active in Douma that reported none harmed by chemicals) they were speaking to in order to get that number:
https://www.rt.com/news/423539-red-crescent-ghouta-no-chemical/
Assad's thing seemed more like either he was genuinely clueless or was playing aloof and also didn't consult Russia before answering, which is odd. Like "Oh, people are saying there was chemical gas? Well I didn't use any, but they had used some on us before, so if there were chemicals then the militants probably had a storage that got hit in the bombing."