Well, that was more or less my (badly expressed) point.
Let's say this computer is connected to another computer via NetBEUI. If that other computer is exposed to the Internet, then this one is potentially exploitable too.
If there is an Internet connection anywhere within a group of devices using some sort of shared communication protocol (or group of protocols), then all the devices in the group are vulnerable. Sure, an attacker would likely need reasonably detailed knowledge of how the devices communicate, along with knowledge regarding how to exploit each step of the chain... but we're talking about targets in the Pentagon. A foreign state actor would have the resources to throw at the problem.