It's perfectly legal to buy, make or posses an airgun silencer. It must be made specifically to fit an airgun and only intend for that purpose. The reason there is conflicting information on this subject is because a few in the airgun business don't understand the laws and have spread completely inaccurate information.
According to the ATF a silencer can't be removable or must be an integral part of the airgun. Otherwise it could be considered a firearm silencer under certain circumstances. Even within those guidelines, attempting or intending to remove one for use on a firearm is still illegal. This means if you have any type of silencer on or off of an airgun and attempt or intend to use it on a firearm you are breaking the law. There would have to be evidence you intended to use it on a firearm. It would have to fit a firearm you had access to or they would need evidence you intended to make it fit. There have been cases where this happened. In those cases the offenders were under investigation for serious criminal activity. They were also in possession of or attempting to acquire a firearm and an airgun silencer.
To put this in clearer context, if you're in possession of an automotive oil filter, you could be arrested and prosecuted for possession of an illegal silencer. There would have to be evidence you intended to attach it to a firearm. Oil filters are widely known to make very effective silencers. Any object or material that can reduce the report of a firearm could be considered an illegal silencer. It's what you're doing or intending to do with it that determines if it's illegal or not.
If you're not engaged in criminal activity, have no related criminal history and aren't planning to mount an airgun silencer on a firearm then you don't have much to worry about. Just don't do something stupid like carry or store an airgun silencer (or oil filter) with a firearm. That could look pretty suspicious.