I'm not much of a car person, but did limited slip differentials exist at the time this video was made? To me it would seem more odd if an advertisement explained the mechanics behind something that the product it was advertising couldn't do...
Limited slip differentials were apparently invented in the 1930s but were relatively exotic (reserved for race cars) until about the 50s. I suspect the makers of the film were just a little confused and didn't fully appreciate the problem.
