How is the title misleading? It's exactly the claim the article/Fisk makes.

The doctor’s testimony is consistent with that of the well-known Syrian opposition group Syrian Observatory for Human Rights (SOHR), which initially reported based on its own pro-rebel sourcing that heavy government bombardment of Douma city resulted in the collapse of homes and underground shelters, causing civilians in hiding to suffocate. [...] Fisk details the Syrian doctor’s testimony, who is adamant in his emphasis that civilians were suffocating en masse, and were not gassed: It was a short walk to Dr Rahaibani. From the door of his subterranean clinic–“Point 200”, it is called, in the weird geology of this partly-underground city–is a corridor leading downhill where he showed me his lowly hospital and the few beds where a small girl was crying as nurses treated a cut above her eye. “I was with my family in the basement of my home three hundred metres from here on the night but all the doctors know what happened. There was a lot of shelling [by government forces] and aircraft were always over Douma at night–but on this night, there was wind and huge dust clouds began to come into the basements and cellars where people lived. People began to arrive here suffering from hypoxia, oxygen loss. Then someone at the door, a “White Helmet”, shouted “Gas!”, and a panic began. People started throwing water over each other. Yes, the video was filmed here, it is genuine, but what you see are people suffering from hypoxia–not gas poisoning.”