The article clearly states that the classified network is physically separated from the internet. All the super sensitive info is on a network COMPLETELY SEPARATE from the rest of the world. Furthermore, in order to "connect the laptop to the network" would require significantly more than plugging in the cable and accidentally leaving wireless enabled. Ever heard of port security? How about MAC filtering? There would be no accident for a system to be attached. It would HAVE to be malicious, and even then the individual would have to have access to the router and network configuration to get it to connect. My guess is that most of what they stole was either non-vital or completely falsified. Just because other nations are working on the development, doesn't mean everyone knows everything about the plane, thereby giving everyone access. No matter how much of a "joint" or "coalition" effort it is, there are parts and pieces kept from other nations, especially if it is a US led effort. Have you told your friends ALL of your secrets? The government doesn't either.
Stop friggin worrying!