I believe that they perceived it in the numbers. Exploding bombs will kill a limited number of people. Imagine the nightmare scenario that a bat-bomb will explode in a school and kill 10? 20? toddlers. Now imagine some virus that can wipe hundreds/thousands. This escalation would force the opponent's hand to respond appropriately, and if 'standard' war (bullets, artillery shells, missiles) is hell, chemical warfare is worse.