Not a conspiracy theorist, but I disagree with that statement and logic.
If you blow up a big building, it's going to scare people who live in areas with dense population and live/work in big buildings. While most Americans live in an "urban/suburban" setting, terrorists are pretty unlikely to come and blow up a building in Iowa City or even a house in Queens because the impact isn't quite as dramatic as brining down a building with thousands of people inside.
If you hijack and airplane and blow up a big building, it has the potential to scare a larger audience. About 1.7 million Americans fly every day, including people who live in suburban and rural environments. Many flights connect through major airports in major metro centers. It's an added layer of terror. Just look at how flying has changed since 9/11 (arriving at the airport far earlier, creation of entirely new government departments, Air Marshals, stricter limits on what you can carry on the plane, body scanners and pat downs, etc.). This has impacted many, many more Americans than just those who live/work in dense urban areas.
Also, they tried to simply blow up the WTC with a bomb in 1993. It was a bit of a failure, but what attack would've caused more fear? One with a bomb in a truck that brings down a building? One with strategically placed structural bombs that brings down a building? Or one where planes originating from three different cities (Boston, Washington D.C., Newark) all converge to bring down a building (+/- additionally placed explosives that may help ensure the result you wanted)?