Well, they evidently did a survey of a sample of gun owners to estimate how guns are transferred or obtained. The accuracy would depend on a lot of factors such as the sample size, the representativeness of the sample, etc... But it is possible to estimate, just like political polling or surveying defensive gun use.
Information gleaned from news reports leads me to believe that gun control advocates are questioning whether or not they would be able to pass a straight up "assault weapon ban" or possibly even a magazine ban, so they are gearing up to also push for banning private sales and keeping a database of all sales and transfers - effectively creating a national registration of guns. That is their Holy Grail anyway as once they have that they can push piecemeal or wholesale gun bans - or so they seem to believe - and will know who has the guns to turn in.