I think the idea is to use simple physical media, like CDROM, where hiding a malicious payload is far more difficult since there's no embedded electronics on a disc.

If that's too cumbersome, one could always implement a simple teletype-like system with a low bitrate. Like an automatic machine that can physically press keys on a keyboard. Then remove every key except letters, numbers, shift and tab.

You could use a similar system or OCR to get data off the gapped machine.