That percentage goes up and down depending on people's willingness to admit that to a nosy surveyer; RKBA activists attribute this recent increase to the huge numbers of first time owners who haven't yet decided on a policy of discretion. Me, I wonder how many of us longer term owners have adopted a FU attitude in the presidential campaign with the starkest choice ever presented to us, excepting perhaps G. H. W. Bush vs. Dukakis, but Bush the father was much closer to total prohibitionist Dukakis, and in fact we're still living with his import bans....
You are of course right that a list of suspected gun owners could be generated in a whole bunch of ways, although I note that when it comes to online ammo dealers I've never seen a group of vendors who are faster in discarding their records of past sales.
Obviously there is a possibility for a database loss if they tracked all gun sales, but considering the stat that 30% of households have a firearm and the many other possible associated data sets the risk seems minimal.