I strongly suspect that they very much ARE air-gapped. That's one of the most cost-effective approaches to a viable IA Case in very-high-integrity systems. If it weren't for one thing* I could name you well over a dozen UK state/military systems which run in an air-gapped environment for just this reason. These systems are not connected to external systems, ever, except by a form of data-diode for status indication. If you want to use/maintain/update them you must physically touch the actual systems.
PDR
* The "one thing" being that as you would expect for these kinds of systems one is not actually allowed to name or discuss them!