No the original poster is quite correct. The Prius relies on the fact that the car is stopping and starting to lower the long run (average) energy requirements of the car. Hence you can use a smaller power plant and supplement the higher instantaneous energy demands with the electric motor driven from a storage battery. However when travelling at speed the average power requirement goes up and the IC has to work harder. If you drive conservatively (ie slower and very smooth) you don't overtax the IC and run it in an inefficient mode. If you try to keep up with every other car then that small power plant coupled with heavy batteries become a significant disadvantage.
By comparison my large Citroen C5 station wagon averages 5.6l/100km on long runs in summer with the a/c running over rolling hills without me being very careful*. My C5 is a much bigger car that is well within the margin of the Prius' efficiency on highway cycle, but worse round the town ~8.2 l/100km around the city for the last 3000km. As you will notice I get a significant increase in efficiency between city and highway driving, as all IC cars do. They are designed to perform well at high speeds and do OK around the city (people like fast powerful cars). This doesn't happen with the Prius, is can actually be the other way around.
More generally, if you look at other comparable small cars they do significantly better than my car and seriously embarrass the Prius. The Prius may look good in the US when compared to a SUV, but they suck in comparison other small cars and then there is diesel.
* Remember a single persons experience does not make a data set.