What the writer is leaving out is the kind of internet data-mining the government is capable of.
It would be easy enough for a relatively skilled programmer to write a program that would troll websites for the user ids of participants claiming to have guns. Then issue subpoenas to the forums’ owners (or to various ISPs) to connect the dots and come up with a real name for the alias.
Most discussion forum owners will fold like cheap lawn chairs, others that resist will manage to draw the process out but only to the point where they’re either bankrupted by legal expenses or the Feds just come in and confiscate the servers.
Once the Feds have real names, they can start launching selective and highly-publicized raids, followed by offers of amnesty to anyone who voluntarily turns in their firearms.
Repeat this process a few times and all but the most dedicated gun owners will be turning in their guns.
It’s just a variation of how RIAA has gone after peer-to-peer music and movie downloaders.
As to the rest ... the Feds will start offering substantial bounties, er, “rewards” to people who turn in those they believe to be in possession of illegal weapons. Every neighborhood has a Gladys Kravitz or two, right? Not to mention the usual neighborhood feuds over tree branches, leaves, barking dogs, etc.
― Aleksandr I. Solzhenitsyn
But if each freeper said they owned a gun.....
;)