The ship canal did not create a footballing rivalry, it created animosity between the cities of Liverpool and Manchester (Salford). This resentment regarding jobs was very real at the time and if you watch the attached MSC documentary that was filmed in the 1970's, the Salford dockers have nothing but scorn for the Liverpool dockers. It's a great bit of footage.
Also, the early 70's was 45 years ago. This is not by any stretch a 'modern' rivalry. And it wasn't created by Alex Ferguson, either. It's one thing making statements of fact, as a respected historian, but when you merge these with your own opinions, then this can sometimes come unstuck. Especially when people who were living and breathing in those decades have the gift of recollection.
Fast forward the clip to 4 mins. As a historian you will enjoy it. Very insightful.