To me this looks like a great example of a strategic information operation. The supposed "strikewarn" to Muslims living in America could be calculated to heighten the possibility of mistreatment of those folks at the hands of the government and public, possibly resulting in further radicalization of at least segments of that population. The announcement of another major attack, with the added factor of possible WMD, is a strike at the nation's morale, hoping to further exacerbate political divisions.
That crossed my mind as well. But then I thought that that strategy would work against radical Islam in that it would presumably unite the rest of us, and put the politically correct garbage (a la McCain and company's bogus Geneva Convention concerns) to rest.
But, at the same time, this is getting so little (actually almost no) press that that consideration is all but moot.
You may be onto something, at least in part, because Balkanization is one of the most powerful strategies in their repertoire -- and is also being embraced by the American left, for politically opportunistic reasons (and the heck with the safety and sovereignty of our republic).