A telescope looking through a proper window at nighttime could be enough. Some LEDs are powerful enough to illuminate a large chunk of an otherwise dark room.

I think it's common to have multiple systems on different airgapped networks in the same room. So if one of the networks were compromised this could let you pivot to another network. Or if they're both compromised it would give a way to exfil from one to the other.

This is what I am wondering. This attack has been known for quite awhile.

That being said it might be enough just to compromise ANY system within the air gapped network - and then escalate from there. Data could still be routed through the computer with line of site (although now we are talking about an increasingly sophisticated automated hack)

Right. In the SCIFs I have been in, if there even are windows, the blinds remain drawn at all times.

You could potentially use a UAV for a sophisticated cyber attack on a specific system.