Well, you can make chlorine gas with common household bleach and napalm with gasoline and Ivory soap. There were a number of individual WW2 air raids that killed a lot more people with "simple" explosives than did the 2 nukes dropped on Japan. There are atomic artillery shells in the US Army inventory that have less destructive power than the OK City bomb.

My personal opinion is that the term "Weapons of Mass Destruction" is entirely a matter of intent. Maybe the intent of the politicians who use the phrase is the important thing.