The extra route the author added took those choosing it from one bottleneck to the other. Who would build such a road and who would choose it as their path? To me this entire analysis is the equivalent of treating traffic flow as pressure in a pipe. Although not a bad first approximation, the behavior of mindless fluid is not an accurate predictor of human behavior. For instance, if one had added a 100 mile long bypass to avoid 1 mile of road, one would slow the net flow of fluid as some of the mindless fluid would spend an inordinate amount of time on the bypass. But people are not mindless and virtually 100% would ignore the 100 mile bypass, as I believe they would avoid the new road taking one through both bottlenecks.