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.
The last few years have shown me something about human behavior.
Before the thing happens: "This will never happen, it can't happen, you're making it up, stop being a scaremonger".
10 minute after the thing happens: "Of course, this always happened and it's always been this way and we just have to live with it".
We are quickly adaptable, but that may be risky if we snuggle up with death.