I suspect that most of their targets use Windows, so we have more malware samples from Windows hosts. The article specifically mentions that they heavily suspect that a Mac malware exists, yet they haven't found it yet. Consequently we don't know which exploits they use to get on Macs. They also talk about a wide range of servers being infected, only attacking Windows servers would be very limiting.

My best guess is that if you are a target, using Linux makes their work harder but not impossible. They had over a decade to figure this out and vulnerabilities in Linux software are found all the time.