It is not just that it is a bugaboo.

Imagine if the material sent in the envelope was from a real nuke, and was therefore verifiably weapons grade. They could claim this, and this claim is readily verified from the observed radioactivity per unit weight.

Now in point of fact, they might not be reasonably able to create a nice city-levelling explosion. But proof that they actually have a source of the necessary materials (the single biggest obstacle for the do it yourself approach to building nukes) would have tremendous impact.

Cheers,
Ben