Because he never specifys what type. It could either specifically mean SLC, or it could mean a city the size of SLC. Theres a myriad of possibilitys to what Chris Avellone meant about those.
When he says "they cover the Salt Lake City area like blankets," it sounds an awful lot like he means they do actually cover it like blankets. I imagine if he was talking hypothetically he would say "
could cover the Salt Lake City area like blankets," or "
could cover
an area the size of Salt Lake City like blankets."
But, of course, you could always just shoot his http://www.gamesas.com/index.php?/user/591393-chris-avellone/ here a PM and ask him yourself. Or ask him in the Meet the Devs thread. I'm sure he wouldn't find it too much trouble to answer a quick lore question, provided he remembers what he really meant when he wrote that.