Finding an original copy on a go-pro would likely be pretty compelling evidence but this (and the more scary politically centered questions like this) are why I wish we had a way to build a durable chain of custody into these technologies. It is infeasible from everything I've seen but it would be a big win for society.
Do you? Consider for a moment all the dissidents and protestors who would be ensnared by their own devices then, with no "it was all ai" defense available?
I don't think the lack of a durable chain of custody really provides any protection - that protection needs to come from a strong legal system and social contract to protect whistleblowers. If you're thinking of, as an example, an Iranian smuggling out protest footage, they're already taking an extreme risk and have a state using numerous tools to try and track them down - but the lack of a durable chain gives a wide area of authorities to cast doubt on the truth.
I think your question is interesting to ponder and I think there are arguments in both directions - but my mind keeps coming back to the tank man photo being smuggled out of China and how much more difficult it would be in the modern world for a single image to carry such weight.