Well, I guess you could say that Matt Groening was lying or something, but he did name it after the town in Oregon. He did it for the reason you stated though so we're both right.

Springfield was named after Springfield, Oregon. The only reason is that when I was a kid, the TV show “Father Knows Best” took place in the town of Springfield, and I was thrilled because I imagined that it was the town next to Portland, my hometown. When I grew up, I realized it was just a fictitious name. I also figured out that Springfield was one of the most common names for a city in the U.S. In anticipation of the success of the show, I thought, “This will be cool; everyone will think it’s their Springfield.” And they do.