In a criminal charge, it really doesn't matter what the popular definition of WMD is, anymore that it matters what the normal usage of "relevant" or "hearsay" is in the context of a trial.
According to the statute, 18 USC 2332a includes a "destructive device," and using the definition cross-referenced in 18 USC 921 (the firearms statute), that means: "any explosive, incendiary, or poison gas -- (i) bomb, (ii) grenade, (iii) rocket having a propellant charge of more than 4 ounces, (iv) missile having an explosive or incendiary charge of more than 1/4 ounce, (v) mine, or (vi) device similar to any of the devices described in the preceding clauses."
It does not include any device that is not designed or re-designed for use as a weapon.
So, any incendiary device -- including whatever kind of bomb you can make -- is a destructive device under section 921, and is considered a WMD if it has a capability of injuring, maiming, and/or killing a large number of people at detonation. It does not include a car (which isn't designed or re-designed to be a weapon, even if you use it to mow down 30 people), unless you pack it with explosives and detonate it, or a firearm (no matter how many rounds it carries), which is defined separately in the same statute.