The problem with this approach, as has been already pointed out, is that no one and everybody is supposed to be Anonymous, so such denial has very little meaning. As Monica Pignotti rightly points out:
"In the end, a group is nothing more than a collection of individuals and since the individuals who make up the undefined group "anonymous" are not identified, there is no accountability or way to tell who did what, unless the individuals who did this are caught red-handed. Even then, since anonymous has no formal organizational structure, it would be difficult to prove any kind of affiliation. Its the perfect set-up to get away with just about anything. Interesting to observe how anything that conflicts with a certain image of anonymous as nonviolent gets explained away -- the creation of a nonfalsifiable argument. I doubt this is convincing to anyone but the believers, however."