Guns can wear-out, but they generally last a long, long time and are the kind of thing that you pass down to your kids and grandkids. A lot of guns are kept for sentimental reasons and may not be something you’d take out to shoot on a regular basis. The Guardian is probably counting the people that say “Oh yeah, we have a gun in the house.
Grandpa’s old Kentucky rifle is hangin’ over the fireplace”.
The majority of states have no gun registration, so they really have no idea who owns guns and how many.