Weapon of mass destruction? A van bomb is a dangerous and serious weapon, to be sure, but since when does it qualify as a WMD? Unless you were to use it to strategically destroy the support frame of a bridge or building and bring about a collapse, or detonate it in the middle of an orgy, WMD doesn't seem an appropriate label - or are there certain legal definitions now, perhaps that define a certain explosive power? The article doesn't mention how powerful this kid thought the bomb was going to be.
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.