Of course in a darkroom someone skilled could always make a fake photo - but the bar is a lot lower with AI.
Or they just point a camera at a high resolution playback of a forged video.
This also assumes you can trust all the camera makers, because by doing this you're implicitly giving them each authorization to produce forged videos. Recall that many of these companies are based in China.
It seems like it would just be a mechanism to lend undeserved credibility to something that still isn't trustworthy.
Exactly, it's just like DRM or cheating in video games. Even if everything in software is blocked, there's always the analogue hole.
Rather than cryptography which could be difficult to grok for a non-technical jury, a physical slide of film would be the source of truth.
It can still be faked by photographing a manufactured/generated scene though.