Well it occurs to me that by denying these claims they are generating more hype than they would by flat out saying "oh yeah, that's what it is" and then putting an end to the buzz because everyone will just wait until the annoucement is made with far less anticipation.
If I had to guess at what they are pulling, given the theme of the games, is that they are denying the existance of the upcoming game/annoucement, just like the goverment would deny the ops. the game is based on, despite all evidence to the contrary.
Also if we're looking at a spin off game, they could technically be telling the truth. You know, a CoD game, set in the CoD universe, but not actually called "Call Of Duty".
It could also be a phrank as well by some bored fanboys, I mean it's not expensive to make dogtags and such, plenty of people offer the service. Buy a few dozen of those as custom jobs, set up a site, and pay some postage to send them to the media, massive lulz ensues from trolling one of the biggest game fan bases out there.