The US legal definition of WMD allows this sort of strange charge, that much I do know; what the definition is, I do not know. Certainly, though, it is a legal definition thing.
The relevant section of the linked Wikipedia page for WMDs. When conventional weapons are involved there are measurable amounts of destructive force in the definition. Just from the numbers I don't know how much damage those values would cause, though.
This was, as far as he knew, something that was going to take out a city plaza during a public event. The WMD charge is probably going to throw a jury for a loop, too.