"The mi2g study concentrated on 'overt digital attacks' and didn't include more general forms of attack such as viruses and worms."
In short, Windows can be violated automatically and rampantly through viruses and worms while Linux requires an "overt digital attack."
Perhaps Windows would be violated more frequently through "overt digital attacks" if it wasn't so overwhelmingly easy to compromise it with a worm or virus.
Anyway, you can harden Linux. You can firewall, patch safely, restrict permissions, and enable SELinux. What choices do you have for Windows?