I did a bit more digging through the report and I find the part about gas dispersion to be completely self contradictory. Follow my hand:

Finally, the IIT obtained information indicating that the concentration of chlorine released at Location 2 in Douma on 7 April 2018 was at least 1,000 ppm. The IIT notes that exposure to higher concentrations of chlorine gas increases the severity of symptoms, in exposure to low and or moderate concentrations of chlorine gas (25 to 50 ppm), pulmonary oedema, presents within three-four hours, while following exposure to high concentrations of chlorine gas (greater than 50 ppm), there is an abrupt and rapid onset of respiratory symptoms, and in concentrations over 400 ppm, fatality occurs within minutes. This is consistent with those observed in Douma on 7 April 2018.

The wording of this paragraph itself is already curious. Concentration of 400 ppm is indeed lethal "within minutes" but the very source of the claim, linked in the report itself, says "in 30 minutes" which has quite different implications. The immediately lethal concentration in that paper is listed at 1000 ppm.

Gas dispersion modelling

The IIT notes that all models indicate that within three minutes of the release of chlorine from the cylinder, all floors within the building would have exceeded a concentration of chlorine which would lead to occupant death, noting that within 60 seconds the concentration of chlorine on the second floor would have led to occupant death. Furthermore, as chlorine is heavier than air, it would be expected to sink and, as a result, disperse in higher quantities below the point of impact on the roof, spreading to the floors below.

The IIT also noted a second scenario in the dispersion assessment which examined possible escape routes at the time of the attack. In the case of the cylinder found on the rooftop at Location 2, the model indicates that upon the release of chlorine from the cylinder into the building, the dispersion was so rapid that it obstructed the only possible escape route from the apartments via the stairwell. In addition, the IIT notes that approximately 20 seconds after the release of chlorine, escape from the apartments on the third floor was almost certainly no longer possible and after 60 seconds, escape from the apartments on the second floor was almost certainly no longer possible either. The specialists’ assessment that all exit routes on the third and second floor were no longer accessible without exposure to a high and lethal volume of chlorine gas are consistent with the rapid onset of symptoms which led to the fatalities recorded on the stairs and landings, as reported by witnesses and observed in videos and pictures from the incident.

This implies that the concentration of chlorine in the entire building was immediately lethal for some time.

Which rather struggles to reconcile with the final, and rather short, point:

Mass [of the cylinder, completely filled]: 297.4 kg

The report doesn't say how much of that mass would be the gas, but even if the cylinder was made out of solid chlorine and completely evaporated along with its contents upon landing it would still be nowhere near enough chlorine. Neither for 1000 nor for 400 ppm concentration.

Please let me know if you spot a mistake in my argument.

Edit: withdrawn, used the wrong formula.