ATF Online - Firearms - Guides - Importation & Verification of Firearms, Ammunition and Implements of War - National Firearms Act Definitions - Silencer
Fake suppressors do not reduce the sound signature of the firearm at all, so they do not fall under this classification. Remember that manufacturers have to follow Federal guidelines very carefully. They aren't going to sell something that falls under the NFA as a non-Class 3 item, they would have agents at their door in short order.
Sound can even be redirected and not fall under the NFA. The Noveske KX3 is a good example. Even though the sound signature is audibly reduced to the sides, the entire signature is redirected downrange and is not reduced by any measurable level. Thus, it is classified as merely a
flash suppressor, not a sound suppressor.
That said, you still have to consult your state regulations to make sure they are legal in your area. When in doubt, ask and confirm.