That is not air gapped

The original post said "air gapped environments", not "air gapped computers". Running several computers on a network which has no connections to the outside qualifies as an air gapped environment, and will let you use a key server just fine.

My assumption is that the system is on an air-gapped network, as individual systems that are completely isolated are typically not very useful as a full user environment, and are more likely to be fully embedded systems instead.