He says that by virtue of that fact that his audience is mostly using Chrome and macOS, they are privileged. Therefore anyone who isn't is therefore underprivileged.
Additionally he refers to it as macOS which pedantic and yet incorrect unless the majority of his users are running a beta OS.
I think, the author meant that (most) web developers consider themselves privileged, in thinking that all developers should be using MacBook Pros and Chrome, and that there’s no need to test in anything else, when sharing demos and tools between themselves.
Using the term macOS is fine. It’s the new name, and we can start using it before the official release of the next version in the fall. I think you are being pedantic here.
