This is obscenely true and somewhat underrated.

Sad as it is, the prime reason for the strength of DoD computer systems, both afloat and in the air, is that it's difficult enough to use and interface with them-- 'Hacking' them, and getting the ability to pivot and deny/destroy is extremely limited. Sailor Joe Schmuckatelli's Fire control system is on a separate network because there was no common network at the time the contract was let for competition. It's both the worst and most extreme version of security through obscurity. Now you're introducing more connections, availability, and reliance on a core infrastructure-- Which still won't meet basic corporate cybersecurity / patch control doctrine.

I can't help but think of:

"The ultimate cause of our failure was a simple one: despite all statements to the contrary, it was not due to lack of bravery on the part of our men, or to any fault of the Fleet's. We were defeated by one thing only - by the inferior science of our enemies. I repeat - by the inferior science of our enemies."

The U.S. military has been very aware of this threat for a few years (though a legacy of disregard precedes that, as they saw no peer threats and were fighting low-tech insurgents). They're not idiots; it's obvious and they hack into their enemy's systems. In fact, they have been focusing on systems and training that enable units to operate while disconnected from the defense networks.