...diminishing the report of a portable firearm...
It is my understanding that this section is interpreted to mean that nonportable items (like shooting "tunnels" with sound-proofing or other large and nonportable sound-absorbing chambers that could be shot through to muffle the report) are not silencers.
Otherwise, an indoor shooting range would be a silencer because it reduces the report of firearms fired inside it as far as the people outside it are concerned.
So, while a silencer does not have to be attached to a firearm to legally be a silencer, it does have to be attachable
to a portable firearm for it to legally be a silencer.
The linked thread contains a letter from the BATF that addresses this topic.