Military networks are fairly secured, while the article does bring a few interesting anecdotes it's quite hyperbolic.
Also I would like to point out that every big hack today, especially those from state actors used fairly common and even "outdated" attacks and vulnerabilities, they were masterfully executed but it's not like they used some super new exploits and attack vectors that no one heard of before.
Military doctrines take time to develop, even in "cyber warfare", military computer systems also don't tend to be the latest and greatest as far as hardware and software goes but they are often reliable.
Building defenses around what you know and what your attackers are able to do is a good strategy, knowing when and what to airgap and what to protect is also critical because not every military network is secret nor critical.
You focus your resources on an effective strategy the 80–20 rule applies even to military and defense networks.