That's a broadening of the term WMD in the US civilian and criminal legislation, basically defining any "destructive device" as WMD so that anyone setting off hand grenades or pipe bombs can be charged under their extremely severe terror legislation.

The US military, like every other organisation outside the US, doesn't include conventional explosives under WMD.

That's incorrect. CBRNE (Chemical, Biological, Radiological, Nuclear, Explosive) is literally in units' names.[0]

0:https://www.army.mil/20thcbrne

The first definition line comes from a definition of a weapon of mass destruction as related specifically to criminal terrorism charges in the US federal code. In this case they are basically setting the lower bound for what could be classified as a 'bomb'. Nothing more. The WMD definition you are probably thinking of (and the one which is more relevant to the original article) is the military definition outlined in various conventions and treaties.