I think the reason the negative luminance is potentially important for secrecy is that it means the average of the signal you’re transmitting is zero, making it indistinguishable from noise.

Or you can call it encryption along different axis. Much like extracting GPS signals from below thermal floor level - you can do it if you 1) know it's there, and 2) know exactly how to key in. It's impressive as heck, but you can always rephrase it in terms of information theory in ways that makes it sound like slightly different shade of mundane.

I don't believe you're missing anything. This is just stegenography with a possibly new covert channel, right? Apparently the secret depends on advisaries not noticing the special hardware deployed on each end. Would using spread sprectum techniques would work just as well?

Yeah, but saying that doesn't get the military to give you money.

Yep.