But to simplify, cluster mutinous are more effective vs troops than a single large detonation. Similarly, rather than starting from a single source you want to spread the weapon from a large number of points. By comparison to 7mg/m3 a 1 metric ton weapon could ideally cover a square kilometer to a thickness of 30m and concentration of 30mg/m3.
Notably, exposure time plays a major role in lethality so covering a wider area makes it much harder to escape from.
Estimates of the death toll range from at least 281 people[3] to 1,729.[14] and 3,600 patients displaying neurotoxic symptoms in 3 hospitals supported by MSF[5]
However, it was considered safe enough for the UN to inspect the area the next day.
Also you're badly overlooking the obvious. In full "...killing 12 people,[1][2] severely injuring 50, and causing temporary vision problems for nearly 1,000 others"
Bret gives the example of the Tokyo terrorist attack that released sarin in several crowded subway cars... and killed 12 people. Grenades would have killed far more people far more quickly... and that's the point: chemical weapons are far less effective at delivering death than explosives are.
Edit: Wikipedia gives 7mg/m³ for a lethal concentration for one of the Novichok agents, 28mg/m³ for sarin.