A critical mass of u-235 is not a bomb.
from wikipedia
http://en.wikipedia.org/wiki/Little_Boy
the author of the email probably used this source.
Credible? I dunno.
Nevertheless, the writer's point remains valid, if awkwardly expressed.
I.e., we should not exclusively concern ourselves with the delivery or a nuclear bomb, or missile. The import and fabrication of a mere "device" is the more likely delivery system.
And the essence of that is: hermetically sealing the borders to that degree is impractical, if not impossible. Once Iran has the ability to refine weaponized uranium, we can no longer rely on defensive measures.
Consequently, it becomes necessary to take the offensive initiative.