The point is that in the linux world you're choosing between competing distributors. In your example, they're both from Apple.
In the linux world you get a combination of 1. low switching costs between competitors (because, as you point out, most of the software is the same), and 2. competition between distributors. With Microsoft and Apple, if you want to choose between competitors, the cost of switching between them is much higher.
In the linux world you get a combination of 1. low switching costs between competitors (because, as you point out, most of the software is the same), and 2. competition between distributors. With Microsoft and Apple, if you want to choose between competitors, the cost of switching between them is much higher.