A. He's arguing against a straw man: "six million Jews were gassed." No reliable source claims that, because Jews and other victims of Nazi atrocities were documentably killed in other ways as well.

B. He also makes the claim that since the room is not hermetically sealed, it couldn't be effectively be a gas chamber. This is logical nonsense. If you breathe too much poisonous gas, it'll kill you. A great enough concentration of gas in a place where people are will kill them. If you keep them breathing air with high concentrations of carbon monoxide long enough, it'll kill them; so, if you posted armed guards outside a room filled with people and ran an furnace exhaust into it, it would kill them too-- consider carbon monoxide poisoning in houses, which are not sealed either. The area does not have to be hermetically sealed as long as there is enough gas. The hermetic sealing in modern gas chambers has an additional purpose-- keeping other people from breathing the gas.

No, I don't know if the door to the crematorium was blocked up. If one used enough gas, it wouldn't matter.