It has nothing to do with a clean engine. I just bought a 2003 Focus with only 80k miles and it's spotless through the fill hole. However, the records show that an EGR valve needed to be changed. Why? Because during short trips the engine doesn't spend much time at operating temperature, so it produces more soot and other emissions. This will affect emission equipment and oil, so changing oil is critical in these types of scenarios.
Highway miles mean that the car spends most of its time at a steady speed, RPM and operating temperature. These parameters mean that the engine produces the least amount of contaminants and emission particles, which in turn doesn’t clog catalytic converters, EGR valves and leaves the oil cleaner.
In reality the engines will last probably the same with either driving cycles, provided proper maintenance was done, but it is the emissions equipment and suspension that will see the difference.