Sure, but in that case your chain starts with your camera, producing "pro" looking video which would be totally unlikely (not mentioning trivial stuff like noticing monitor/lens distortions etc.). Analyzing subsequent processing steps stored in blockchain would likely show very low probability of authentic work due to missing many required steps (OK, there is still certain low probability you can fool it somehow, but would you want to waste so much time/processing power on it?)

I believe that the usual response to this is that the devices include some kind of unmodifiable time and location stamping, although that argument spirals out of control pretty quickly.