If you read the post a bit more slowly and carefully, it may become clearer that the latter part of the is about scientific computing in general.
As it happens, I work in a high performance computing center and use Linux all of the time. Beyond that, Macs account for about half of the personal desktops or laptops used here, with Windows and Linux also represented. What I like best about my OS X laptop is that I can do essentially anything I need to do from it, through it, or via scripts that share between the two.
That having been said, I agree with you that Linux dominates the bulk-computing environments found within high performance computing, despite serious attempts on the part of Microsoft to get traction into this space. To first order, scientific computing IS Linux, with personal machine environments connecting to these systems left to personal choice.
My point remains that Linux is essential for science and that someone going into science will surely be exposed to it and trained in its use. My personal choice for an individual laptop and desktop environment is OS X, and I note that a larger fraction of scientists make this choice than in other fields, as well.