I don’t know too much about image processing, but my intuition tells me that distinguishing a photo of a 3d scene from a photo of a photo of a 3d scene is a much easier engineering problem than distinguishing it from an AI-generated diffusion. So even just reducing the problem to the former is still a worthy accomplishment.
Agree with the weakness, but the benefit of provenance is still there. You can't tell if the picture that person took is of an AI image, but there's some value in saying "I took this" and being able to prove it.
I hope future social media presents this metadata so if someone claims a photo is from the BBC, it actually gets checked against the keys the BBC sign their photos with.