I think the general point stands though. While nothing is perfectly secure, having small and understandable components that are fully audited should allow a high level of safety
If a network stack on a modern computer is too dangerous, then use a modem (silly example: apt install minimodem) and an aux cable from the one computer's speaker to the other's mic jack, or a serial connection (not very familiar with those, can't say how complex the driver is there) or something similarly basic that you can audit a memory-safe implementation of
You advocate for really simple application layer, while having that on top of all the other complex communication layers. Implementations had multiple known vulnerabilities over the years. In case of vulnerability an attacker might be able to do much more damages with real-time access. Is it any safer than an USB stick?
On top of the complex communication layer we're trying to avoid? Umm, I'm not suggesting to run an aux cable or serial connection on top of a TCP stack, so I don't understand what you're saying
Edit: or do you mean the other way around, namely running a network stack on top of this (e.g.) serial connection? Also not what I meant but I wasn't explicit about that so this confusion would make sense. What I had in mind is doing whatever comms you want to do with the airgapped system, like logging/storing the diplomatic transmissions or whatever this system was for, via this super simple connection such that the airgapped system never has to do complex parsing or state machines as it would with something like USB or a standard kernel's network stack
If a network stack on a modern computer is too dangerous, then use a modem (silly example: apt install minimodem) and an aux cable from the one computer's speaker to the other's mic jack, or a serial connection (not very familiar with those, can't say how complex the driver is there) or something similarly basic that you can audit a memory-safe implementation of