I mean, there are all kinds of ways to bound uncertainty. You’re right, you could film another camera, but that could be addressed separately. Digital signing would only prove that a video came directly from a particular camera’s sensors. To assert the veracity of a video, you’d then have to prove that particular camera was at the scene, and the light coming into the lens was authentic. Most court footage is incidental, not intentional, so the sort of doctoring you describe is unlikely in a majority of cases. Where it is likely, different measures could be taken to bound uncertainty.

Separately, generating false images or videos for libel purposes would become more difficult; if a camera that has never been owned by a person is used to produce a signed video that is pornographic in nature, the authenticity of that video is immediately suspect. It would take a much more sophisticated attack to produce a mathematically convincing fake video.