Yes like I mentioned WMD use explosives as a dispersant , theres the confusion. Does not include bombs, IED's which under 921 would be defined as a destructive device 4 - A,i

WMD is often referred to by the collection of modalities that make up the set of weapons: chemical, biological, radiological, nuclear, and explosive

I guess they listed explosive this way is because not all chemical and bio WMDs need to use an explosive device . Some can be just added to the water supply or food supply or just in the air . So to cover all WMDs with or without, the word explosive was added as use of an dispersant.