After reading the article ... it's not clear that the passenger told the newspaper about it directly.

My guess is that the situation happened like this: the passenger got to his hotel room in Chicago, discovered the firearm, and then said "@#$!, how do I get home with this?". Not being an expert on checkpoint procedures (probably because he doesn't read this forum ), he probably called Southwest, which probably started a cascade of phone calls. At some point, a newspaper probably caught wind of the story.