You totally misunderstood the comment.
If all the KVM units were on an airgapped system, there is no way to reach those units other than physically sitting down at the C&C workstation that is meant to interface with them and display their output. Because that machine is also on the airgapped network, and is not reachable from the Internet.
It’s no different than a traditional KVM at that point, aside from that C&C machine being anywhere where Ethernet can reach (traditional KVM units being rather distance-constrained).
Now, if you need mobile/off-site access to this system, you put a second NIC into that C&C workstation. First one for the KVM network, the other for world+dog, and then you use a trusted remote-access system to access the C&C workstation, and block it off from anything else on that second Internet-accessible network as best as possible.
I mean, you want secure? Truly secure? Then disassemble all your computers, put each individual part into its own barrel of cement, and then drop each barrel into its own deep-oceanic abyssal trench. THAT is how you get true security.
For everything else, there are reasonable trade-offs that discourage all but nation-state players or people with wrenches.