Probably. I think part of his point is that these days it's very hard to have networks completely isolated from the internet. If there is only one host connected to both the internet (or a network that connects to it) and that proprietary network, it can act as gateway when compromised and unleash mayhem.
I know Steve read all the top secret manuals when he worked for a military contractor, but is this true? Specifically the part about weapons systems? Doesn't the military have a proprietary satellite communication system for that stuff?