I doubt I'll have much success in this, but I've tilted and windmills before:
Chemical Weapons are indeed "Weapons of Mass Destruction" - and the key characteristic that makes them so is *indescrimination*.
A straight-up HE bomb (or even a pie-in-the-sky KE weapon) has a known blast radius around its intended target. Pick target, apply Circular Error Probable, apply blast radius, and you now have a circle that pretty accurately defines the amount of damage that weapon will do.
With a Chemical, Nuclear, or Biological weapon, that calculation no longer applies. With each, you get a cloud of contamination whose extent and direction you cannot predict, and - as the contamination is persistant to some degree - you cannot predict the number of unintended exposures to weapon effects after the fact.
A single machine gun, or even a knife, given enough persistance and patience, can indeed kill as many people as any CBRN strike. But unlike the CBRN strike, each person killed will have been done so purpously and with intent - and in the occasion of unintended casualties, those numbers will be small. Not so with a CBRN strike on a military target outside a city, when the wind changes and accidentally contaminates a major populated area..
It is that capability to expose large numbers of non-combatants to weapons effects *indescriminately* from actual combatants that makes these "WMDs"