Well that's the nature of trying to prevent a thing that hasn't happened yet right? What would be reliable evidence except that terrorists already used AI to help build a bioweapon? I'm sure before 9/11 talking about terrorists using commercial airplanes as makeshift missiles seemed like scaremongering too.

I see zero technical reasons why it could not be done, so being concerned about prevention seems pretty reasonable. I'm not saying AI uses robotic arms to build a bioweapon unassisted or something, just that it dramatically empowers bad actors enough to make them capable of things they previously were not.