The clue: it depends on the definition of "national security"?
To quote them, "If catastrophic failure of the network is the threshold by which national security threats are defined, Microsoft wouldn't qualify, simply because their monoculture is not at the core of the network," says the George Mason report. "No matter how many Windows operating sytems are infected or fail, the core of the network will still run, even if there is nobody left to send traffic."
Err... I have a headache now.